In recent years the organization of mass spectrometry (MS) data obtained in large-scale proteomics projects became an important issue. This has catalyzed the development of a few different database schemes for storing MS data, as well as some dedicated user interfaces. However, many of these projects are still rather immature and often do not cover all needs. Because our needs were quite specific, it was necessary to build a database that accommodates all the major types of experiments generated in house and that could be easily extended by new modules made by collaborators or students. A database application named “YassDB” will be described in this chapter. The application is implemented in a “three-tier” application architecture, with a database layer, a middle layer consisting of web services and a client layer, containing the user interface. This offers high flexibility: it allows other applications, written in any language, to be written as clients to the database. The setup and use of the YassDB database application with two client programs “pProRep” and “VEMS” will be outlined.