Sequence diagrams are a widely used design notation for describing software behaviors. Many reusable software artifacts such as design patterns and design aspects make use of sequence diagrams to describe interaction behaviors. When a pattern or an aspect is reused in an application, it is important to ensure that the sequence diagrams for the application conform to the corresponding sequence diagrams for the pattern or aspect. Reasoning about conformance relationship between sequence diagrams has not been addressed adequately in literature. In this paper, we focus on required behavior specified by a UML sequence diagram. A novel trace semantics is given that captures precisely required behavior specified by a sequence diagram and a conformance relation between sequence diagrams is formalized based on the semantics. Properties of the trace semantics and the conformance relation are studied.