Electrospray (ESI) is a soft ionization technique largely used for biological sample analysis. ESI-Tandem mass spectrometry (ESI-MS/MS) offers an alternative way for sequencing. For a better understanding, the teaching activity start with an introduction to electrospray ionization technique and collision - induced dissociation. In the second time, it will provide the detailed description of peptides fragmentation rules as read from ESI-MS/MS. To better illustrate the theoretical part, several application exercises will be included.