Feature selection is used to determine feature influence for a given categorization decision to identify those features in a categorized document that were important in classifying the document into one or more classes. In one embodiment, model parameters of a categorization model are used to determine the features that contributed to the categorization decision of a document. In another embodiment, the model parameters of the categorization model and the features of the categorized document are used to determine the features that contributed to the categorization decision of a document.