A versatile library for fitting models directly to visibility data, currently implemented in CASA. These models can depend on frequency and fitting parameters in an arbitrary algebraic way. We have tested the software with both synthetic data and real observations. In some cases (e.g., sources with sizes smaller than the diffraction limit of the interferometer), the results from the fit to the visibilities are far superior to the output obtained from a mere analysis of the deconvolved images. We give some illustrative examples in the software documentation and in Marti-Vidal et al. (2014), A&A 563, 136, arXiv:1401.4984.