Use Cases and Scenarios are sometimes considered as analogous. In this paper, we take the position that use cases and scenarios are different but play complementary roles in requirements engineering. A use case is a collection of scenarios. Use cases are appropriate as specification of a system required behavior in interaction with its actors. A scenario is an example of execution involving a system and its actors. A scenario may be defined with the intention that it should be supported or the intention that it should be avoided. Scenarios can thus be used to validate functional as well as non-functional requirements specification. We present an integration of scenarios to a use case based requirements engineering approach and discuss the merits of such integration.