XLSTAT is a Microsoft Excel statistical add-in that has been developed since 1993 to enhance the analytical capabilities of Excel. XLSTAT is a modular statistical software built around XLSTAT-Pro - the core product of Addinsoft. XLSTAT relies on Excel for the input of data and the display of results, but the computations are done using autonomous software components. The use of Excel as an interface makes XLSTAT a user-friendly and highly efficient statistical and multivariate data analysis package. The quality of the computations is identical to that offered by classic scientific statistical analysis software.