The author introduces transaction based analysis (TBA), an object-oriented analysis methodology that is a blend of use-case analysis and responsibility driven design. The author shows how a user's functional requirements can be captured in a hierarchy of transaction diagrams-this hierarchy is one of the primary models generated using TBA. Transaction diagrams show how responsibilities within the system are carried out-they capture the control flow between objects in the system, as well as indicate which object responsibilities are being invoked. These diagrams also capture patterns of interactions that are intrinsic in the user's conceptual model. The notation for transaction diagrams is robust and easily communicated to the user.