In this paper we investigate the notion of transaction in object-oriented specification. Transactions are a suitable concept to cover different aspects that arise in a variety of contexts. For example, in refinement theory a transaction may be understood as the refinement of actions, in database theory it may be understood as an abstraction from sequences of statements, or in business process modelling it may occur as a complex process with a predefined execution order. Thus, there is a wide application area for such a notion.