Fityk is user friendly, open source software for nonlinear curve fitting and data analysis. It is primarily used by scientists to analyze data from experiments (powder diffraction, chromatography, photoluminescence and infrared and Raman spectroscopy), but can be used to fit any analytical functions to any kind of data. The program allows data visualization, background subtraction, separation of overlapping peaks and least squares fitting. Macros can be used to automate repetitive tasks. Fityk has a dedicated support for placing and fitting bell-shaped functions such as Gaussian, Loretzian, Pearson 7, Voigt, Pseudo-Voigt and Doniach-Sunjic.