Abstract: A playlist engine is provided that selects content items for a partial playlist. The content items may include audio content items (i.e., tracks or songs). In one implementation, a user provides a request to expand a partial playlist. The request may include the partial playlist and an indication of the number of additional content items that the user would like to add to the partial playlist. The playlist engine may select one or more attributes from the content items in the partial playlist, and may use the selected one or more attributes to determine additional content items that are relevant to the selected one or more attributes. The playlist engine may add the additional content items to the partial playlist.