INTerface Builder (INTBuilder) is a fast, easy-to-use program to compute protein-protein interfaces. It is designed to retrieve interfaces from molecular docking software outputs in an empirically determined linear complexity. INTBuilder directly reads the output formats of popular docking programs like ATTRACT, HEX, MAXDo, and ZDOCK, as well as a more generic format and Protein Data Bank (PDB) files. It identifies interacting surfaces at both residue and atom resolutions. INTerface Builder is an open source software written in C and freely available for noncommercial use (CeCILL license) at https://www.lcqb.upmc.fr/INTBuilder .