Recommending content items

Recommending content items is disclosed. In some embodiments, determining an item to recommend includes receiving an indication that a first item is of interest and selecting a second item to recommend based at least in part on a degree of similarity of the second item to the first item as determined based at least in part on a number of other instances in which both the first item and the second item have been of interest and a first decayed popularity of the first item. In some embodiments, the degree of similarity of the second item to the first item is determined based in addition on a second decayed popularity of the second item.