1. A method of browsing a plurality of media items, at least one of said media items being associated with a plurality of tag values, each tag value having a level, the method comprising the steps of: (a) constructing a hierarchical tree structure of tag values according to the levels of the tag values, such that any tag value on any branch of the tree structure from a first tag value is associated with a media item with which said first tag value is associated; (b) displaying at least a first portion of the constructed tree structure in a graphical user interface; (c) changing, in response to a user input, the level of a second tag value in the displayed portion; (d) reconstructing the tree structure according to the changed levels of the tag values, such that any tag value on any branch from a third tag value remains associated with a media item with which said third tag value is associated; and (e) displaying at least a second portion of the reconstructed tree structure in the graphical user interface.