This chapter starts with an overview of the design phase according to the functional and object oriented methodology (FOOM) and presents the subphases and their products. Then, the chapter focuses on the design of transactions. First it describes what transactions are and how they can be identified and extracted from the object oriented data flow diagrams (OO-DFD). Afterwards, it explains how to map transaction diagrams to top-level descriptions, which detail their components and process logic. The transactions’ top-level descriptions will eventually become detailed descriptions of respective class methods. The chapter ends with additions to the data dictionary (DD) the class diagram due to the definition of the transactions.