TOPCAT is an interactive graphical viewer and editor for tabular data. Its aim is to provide most of the facilities that astronomers need for analysis and manipulation of source catalogues and other tables, though it can be used for non-astronomical data as well. It understands a number of different astronomically important formats (including FITS, VOTable and CDF) and more formats can be added. It is especially good at interactive exploration of large (multi-million row, lots of columns) tables.