diff --git a/README.md b/README.md
index 8cdebff..830d475 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,18 @@ There are three kinds of version labels you can use.
 
 * `packages` - Space delimited list of packages to install.
 * `version` - Version of cache to load. Each version will have its own cache. Note, all characters except spaces are allowed.
+* `add-repositories` - Space delimited list of additional repositories to pull from. Examples of potential inputs.
+
+  ```sh
+  'deb http://myserver/path/to/repo stable myrepo'
+  'http://myserver/path/to/repo myrepo'
+  'https://packages.medibuntu.org free non-free'
+  http://extras.ubuntu.com/ubuntu
+  ppa:user/repository
+  ppa:user/distro/repository
+  multiverse
+  ```
+
 * `execute_install_scripts` - Execute Debian package pre and post install script upon restore. See [Caveats / Non-file Dependencies](#non-file-dependencies) for more information.
 
 ### Outputs
diff --git a/action.yml b/action.yml
index 1e77dd8..cdbf092 100644
--- a/action.yml
+++ b/action.yml
@@ -24,6 +24,10 @@ inputs:
     description: 'Enable debugging when there are issues with action. Minor performance penalty.'
     required: false
     default: 'false'
+  add-repositories:
+    description: 'Space delimited list of additional repositories to use. See README.md for a list of examples.'
+    required: false
+    default: ''
 
 outputs:
   cache-hit:
@@ -44,10 +48,11 @@ runs:
     - id: pre-cache
       run: |
         ${GITHUB_ACTION_PATH}/pre_cache_action.sh \
+          "${{ inputs.debug }}" \
           ~/cache-apt-pkgs \
           "${{ inputs.version }}" \
-          "${{ inputs.execute_install_scripts }}" \
-          "${{ inputs.debug }}" \
+          "${{ inputs.add-repositories }}" \
+          "${{ inputs.execute_install_scripts }}" \      
           ${{ inputs.packages }}
         echo "CACHE_KEY=$(cat ~/cache-apt-pkgs/cache_key.md5)" >> $GITHUB_ENV
       shell: bash
@@ -61,11 +66,12 @@ runs:
     - id: post-cache
       run: |      
         ${GITHUB_ACTION_PATH}/post_cache_action.sh \
+          "${{ inputs.debug }}" \
           ~/cache-apt-pkgs \
           / \
           "${{ steps.load-cache.outputs.cache-hit }}" \
-          "${{ inputs.execute_install_scripts }}" \
-          "${{ inputs.debug }}" \
+          "${{ inputs.add-repositories }}" \
+          "${{ inputs.execute_install_scripts }}" \                  
           ${{ inputs.packages }}
         function create_list { local list=$(cat ~/cache-apt-pkgs/manifest_${1}.log | tr '\n' ','); echo ${list:0:-1}; };
         echo "package-version-list=$(create_list main)" >> $GITHUB_OUTPUT
diff --git a/install_and_cache_pkgs.sh b/install_and_cache_pkgs.sh
index f94b87c..e1b7ae6 100755
--- a/install_and_cache_pkgs.sh
+++ b/install_and_cache_pkgs.sh
@@ -3,23 +3,30 @@
 # Fail on any error.
 set -e
 
-# Debug mode for diagnosing issues.
-# Setup first before other operations.
-debug="${2}"
-test ${debug} == "true" && set -x
-
 # Include library.
 script_dir="$(dirname -- "$(realpath -- "${0}")")"
 source "${script_dir}/lib.sh"
 
-# Directory that holds the cached packages.
-cache_dir="${1}"
+# Debug mode for diagnosing issues.
+# Setup first before other operations.
+debug="${1}"
+validate_bool "${debug}" debug 1
+test ${debug} == "true" && set -x
 
-# List of the packages to use.
-input_packages="${@:3}"
+# Directory that holds the cached packages.
+cache_dir="${2}"
+
+# Additional repositories to use for installation.
+add_repositories="${3}"
 
 # Trim commas, excess spaces, and sort.
-normalized_packages="$(normalize_package_list "${input_packages}")"
+normalized_repositories="$(normalize_list "${add_repositories}")"
+
+# List of the packages to use.
+input_packages="${@:4}"
+
+# Trim commas, excess spaces, and sort.
+normalized_packages="$(normalize_list "${input_packages}")"
 
 package_count=$(wc -w <<< "${normalized_packages}")
 log "Clean installing and caching ${package_count} package(s)."
@@ -55,6 +62,26 @@ fi
 
 log_empty_line
 
+# Is length of string greater than zero?
+if test -z "${normalized_repositories}"; then
+  log "Installing software-properties-common package to add repositories..."
+  sudo DEBIAN_FRONTEND=noninteractive apt-fast --yes install software-properties-common > /dev/null
+  log "done"
+
+  log "Verifying and adding additional repositories..."
+  for repository in ${normalized_repositories}; do
+    add-apt-repository ${repository}
+  done
+  log "done"
+
+  repositories_filepath="${cache_dir}/repositories.log"
+  log "Writing added repositories list to ${repositories_filepath}"
+  echo "${normalized_repositories}" | tr ' ' '\n' > ${repositories_filepath}
+  log "done"
+fi
+
+log_empty_line
+
 # Strictly contains the requested packages.
 manifest_main=""
 # Contains all packages including dependencies.
@@ -64,7 +91,7 @@ install_log_filepath="${cache_dir}/install.log"
 
 log "Clean installing ${package_count} packages..."
 # Zero interaction while installing or upgrading the system via apt.
-sudo DEBIAN_FRONTEND=noninteractive apt-fast --yes install ${normalized_packages} > "${install_log_filepath}"
+sudo DEBIAN_FRONTEND=noninteractive apt-fast --yes install ${normalized_packages} >> "${install_log_filepath}"
 log "done"
 log "Installation log written to ${install_log_filepath}"
 
diff --git a/lib.sh b/lib.sh
index c2f1ec9..4abc31a 100755
--- a/lib.sh
+++ b/lib.sh
@@ -118,19 +118,20 @@ function log_err { >&2 echo "$(date +%H:%M:%S)" "${@}"; }
 function log_empty_line { echo ""; }
 
 ###############################################################################
-# Sorts given packages by name and split on commas.
+# Sorts given list by name and split on commas.
 # Arguments:
-#   The comma delimited list of packages.
+#   The comma delimited list.
 # Returns:
-#   Sorted list of space delimited packages.
+#   Sorted list of space delimited elements.
 ###############################################################################
-function normalize_package_list {
-  local stripped=$(echo "${1}" | sed 's/,//g')
+function normalize_list {
+  local stripped=$(echo "${1}" | sed 's/\s*,\s*/ /g' | sed 's/\s+/ /g')
   # Remove extraneous spaces at the middle, beginning, and end.
   local trimmed="$(\
     echo "${stripped}" \
     | sed 's/\s\+/ /g; s/^\s\+//g; s/\s\+$//g')"
-  local sorted="$(echo ${trimmed} | tr ' ' '\n' | sort | tr '\n' ' ')"
+  local sorted="$(echo ${trimmed} | tr ' ' '\n' | sort)"
+  # Echos as an inlined list (not newlines).
   echo "${sorted}"  
 }
 
diff --git a/post_cache_action.sh b/post_cache_action.sh
index 25e7da3..68d9286 100755
--- a/post_cache_action.sh
+++ b/post_cache_action.sh
@@ -7,31 +7,36 @@ set -e
 script_dir="$(dirname -- "$(realpath -- "${0}")")"
 source "${script_dir}/lib.sh"
 
+# Debug mode for diagnosing issues.
+# Setup first before other operations.
+debug="${1}"
+validate_bool "${debug}" debug 1
+test ${debug} == "true" && set -x
+
 # Directory that holds the cached packages.
-cache_dir="${1}"
+cache_dir="${2}"
 
 # Root directory to untar the cached packages to.
 # Typically filesystem root '/' but can be changed for testing.
 # WARNING: If non-root, this can cause errors during install script execution.
-cache_restore_root="${2}"
+cache_restore_root="${3}"
 
 # Indicates that the cache was found.
-cache_hit="${3}"
+cache_hit="${4}"
+
+# Additional repositories to use for installation.
+add_repositories="${5}"
 
 # Cache and execute post install scripts on restore.
-execute_install_scripts="${4}"
-
-# Debug mode for diagnosing issues.
-debug="${5}"
-test ${debug} == "true" && set -x
+execute_install_scripts="${6}"
 
 # List of the packages to use.
-packages="${@:6}"
+packages="${@:7}"
 
 if [ "$cache_hit" == true ]; then
-  ${script_dir}/restore_pkgs.sh "${cache_dir}" "${cache_restore_root}" "${execute_install_scripts}" "${debug}"
+  ${script_dir}/restore_pkgs.sh "${debug}" "${cache_dir}" "${cache_restore_root}" "${execute_install_scripts}"
 else
-  ${script_dir}/install_and_cache_pkgs.sh "${cache_dir}" "${debug}" ${packages}
+  ${script_dir}/install_and_cache_pkgs.sh "${debug}" "${cache_dir}" "${add_repositories}" ${packages}
 fi
 
 log_empty_line
diff --git a/pre_cache_action.sh b/pre_cache_action.sh
index 030f3b5..a37c064 100755
--- a/pre_cache_action.sh
+++ b/pre_cache_action.sh
@@ -1,5 +1,6 @@
 #!/bin/bash
 
+# Fail on any error.
 set -e
 
 # Include library.
@@ -8,32 +9,35 @@ source "${script_dir}/lib.sh"
 
 # Debug mode for diagnosing issues.
 # Setup first before other operations.
-debug="${4}"
+debug="${1}"
 validate_bool "${debug}" debug 1
 test ${debug} == "true" && set -x
 
 # Directory that holds the cached packages.
-cache_dir="${1}"
+cache_dir="${2}"
 
 # Version of the cache to create or load.
-version="${2}"
+version="${3}"
 
-# Execute post-installation script.
-execute_install_scripts="${3}"
-
-# Debug mode for diagnosing issues.
-debug="${4}"
-
-# List of the packages to use.
-input_packages="${@:5}"
+# Additional repositories to include.
+add_repositories="${4}"
 
 # Trim commas, excess spaces, and sort.
-packages="$(normalize_package_list "${input_packages}")"
+repositories="$(normalize_list "${add_repositories}")"
+
+# Execute post-installation script.
+execute_install_scripts="${5}"
+
+# List of the packages to use.
+packages="${@:6}"
+
+# Trim commas, excess spaces, and sort.
+normalized_packages="$(normalize_list "${packages}")"
 
 # Create cache directory so artifacts can be saved.
 mkdir -p ${cache_dir}
 
-log "Validating action arguments (version='${version}', packages='${packages}')...";
+log "Validating action arguments (version='${version}', packages='${normalized_packages}')...";
 if grep -q " " <<< "${version}"; then
   log "aborted" 
   log "Version value '${version}' cannot contain spaces." >&2
@@ -41,7 +45,7 @@ if grep -q " " <<< "${version}"; then
 fi
 
 # Is length of string zero?
-if test -z "${packages}"; then
+if test -z "${normalized_packages}"; then
   log "aborted"
   log "Packages argument cannot be empty." >&2
   exit 3
@@ -55,7 +59,7 @@ log_empty_line
 
 versioned_packages=""
 log "Verifying packages..."
-for package in ${packages}; do 
+for package in ${normalized_packages}; do 
   if test ! "$(apt-cache show "${package}")"; then
     echo "aborted"
     log "Package '${package}' not found." >&2
@@ -74,17 +78,23 @@ set -e
 log "Creating cache key..."
 
 # TODO Can we prove this will happen again?
-normalized_versioned_packages="$(normalize_package_list "${versioned_packages}")"
+normalized_versioned_packages="$(normalize_list "${versioned_packages}")"
 log "- Normalized package list is '${normalized_versioned_packages}'."
 
-value="${normalized_versioned_packages} @ ${version}"
-log "- Value to hash is '${value}'."
+# Create value to hash for cache key.
+value_to_hash="${normalized_versioned_packages}"
+if test -n "${repositories}"; then
+  log "- Added repository list is '${repositories}'."
+  value_to_hash="${value_to_hash} add-repositories:'${repositories}'"
+fi
+value_to_hash="${value_to_hash}@ ${version}"
 
-key="$(echo "${value}" | md5sum | cut -f1 -d' ')"
-log "- Value hashed as '${key}'."
+# Hash the value and set the key.
+log "- Value to hash is '${value_to_hash}'."
+hash_key="$(echo "${value_to_hash}" | md5sum | cut -f1 -d' ')"
+log "- Value hashed as '${hash_key}'."
 
-log "done"
-
-key_filepath="${cache_dir}/cache_key.md5"
-echo ${key} > ${key_filepath}
-log "Hash value written to ${key_filepath}"
+# Write the key out to match for future runs.
+hash_key_filepath="${cache_dir}/cache_key.md5"
+echo ${hash_key} > ${hash_key_filepath}
+log "Hash value written to ${hash_key_filepath}"
diff --git a/restore_pkgs.sh b/restore_pkgs.sh
index fec5b29..b7a8c80 100755
--- a/restore_pkgs.sh
+++ b/restore_pkgs.sh
@@ -3,25 +3,27 @@
 # Fail on any error.
 set -e
 
-# Debug mode for diagnosing issues.
-# Setup first before other operations.
-debug="${4}"
-test ${debug} == "true" && set -x
-
 # Include library.
 script_dir="$(dirname -- "$(realpath -- "${0}")")"
 source "${script_dir}/lib.sh"
 
+# Debug mode for diagnosing issues.
+# Setup first before other operations.
+debug="${1}"
+validate_bool "${debug}" debug 1
+test ${debug} == "true" && set -x
+
 # Directory that holds the cached packages.
-cache_dir="${1}"
+cache_dir="${2}"
 
 # Root directory to untar the cached packages to.
 # Typically filesystem root '/' but can be changed for testing.
-cache_restore_root="${2}"
+# WARNING: If non-root, this can cause errors during install script execution.
+cache_restore_root="${3}"
 test -d ${cache_restore_root} || mkdir ${cache_restore_root}
 
 # Cache and execute post install scripts on restore.
-execute_install_scripts="${3}"
+execute_install_scripts="${4}"
 
 cache_filepaths="$(ls -1 "${cache_dir}" | sort)"
 log "Found $(echo ${cache_filepaths} | wc -w) files in the cache."