This chapter describes the least-square (LS) fitting segment of the program KUPLOT. The user can choose between polynom, Gaussian and Lorentzian for 1D data or a Gaussian for 2D data. Furthermore this version of KUPLOT supports user defined theory functions thus allowing for virtually any fit function desired. A description of the LS method itself is beyond the scope of this manual and the reader might refer to various text books on statistics or numerical mathematics. For a list of all fit related commands of KUPLOT check the online help or the command reference manual. Furthermore, the interactive tutorial contains a fit example.