  TOPCAT is a desktop application for interactive analysis of tabular data,
especially source catalogues. Along with its command-line counterpart STILTS,
it has been under more or less continuous development for the past 15 years and
is now widely used by astronomers from project students to research scientists.
This paper reviews its capabilities as a tool for working with large and small
datasets, and considers some of the issues in design, implementation and user
interaction that have to be tackled when developing software of this kind.
