The MALDI-TOF MS method can be used for the exact determination of molecular weight of peptides. Detailed study of fragmentation of metastable ions after leaving the ion source (post-source decay, PSD) makes it possible to determine also the sequence of amino acids in peptides. If complete primary structure cannot be reliably determined due to the complexity of PSD spectra, the missing part of the sequence can be found in Internet bases. Because of a simple preparation of samples and short analysis times, MALDI-TOF MS is an extraordinarily suitable method for checking purity of peptide preparations. It can be also used for study of peptide derivatization.