Version 3.1 of PicoQuant GmbH's FluoFit multiexponential fluorescence decay fit software for PCs supports Windows XP and previous 32-bit Windows platforms. The software offers a graphical user interface and a graphic installer, and it supports TimeHarp and single-column ASCII data files. It uses nonlinear error minimization based on Levenberg-Marquardt and Simplex algorithms to perform an iterative reconvolution of the instrument response and of the observed fluorescence decay. It supports asymptotic standard error analysis and support plane analysis. The software also can process anisotropy decay and estimation of G-factor.