FITTER is a regression Add-In for MS Excel. It estimates any user-defined model that may be entered in ordinary algebraic notation as a set of explicit, implicit and differential equations. FITTER uses the analytic calculations of derivatives and a special optimization algorithm which provides a high accuracy for the nonlinear models. All calculations are performed in a DLL library created using C++, which provides high speed processing.. FITTER allows you to include prior knowledge about parameters and accuracy of measurement in addition to experimental data. Using Bayesian estimation, you can process both unlimited arrays of single-response data, and data referring to different responses. With the help of FITTER you can obtain a lot of additional statistical information concerning the input data and the quality of fitting. Parameter estimates, variances, covariance matrix, correlation matrix and F-matrix; the starting and final values of the sum of squares and objective function, error variance, and spread in eigenvalues of the Hessian matrix; error variance for each observation point calculated by fit and by population. Moreover, there are hypotheses testing for: Student's test for outliers, test of series for residual correlation, Bartlett's test for homoscedastisity, Fisher's test for goodness of fit. Also, you can calculate confidence intervals for each observation point by linearization method or with the help of modified bootstrap technique.