Fityk is a program for nonlinear fitting of analytical functions (especially peak-shaped) to data. It can also be used to display data or to remove baseline from data. It is reported to be used in crystallography, chromatography, photoluminescence, infrared and Raman spectroscopy and other fields. Fityk knows about common peak-shaped functions (Gaussian, Lorentzian, Voigt, Pearson VII, bifurcated Gaussian, EMG, Doniach-Sunjic, etc.) and polynomials. It also supports user-defined functions. Fityk offers a graphical interface (and also command line interface), various optimization methods (standard Marquardt least-squares algorithm, Genetic Algorithms, Nelder-Mead simplex), equality constraints, modeling error of x coordinate of points (eg. zero-shift of instrument), handling series of datasets, automation of common tasks with scripts, and more.