Web application development using distributed components and web services presents new software integration challenges, because solutions often cross vendor, administrative, and other boundaries across which neither binary nor source code can be shared. We present a methodology that addresses this problem through a formalism for specifying and manipulating behavioral interfaces of multi-threaded open software components that communicate with each other through method calls. An interface constrains both the implementation and the user of a web service to fulfill certain assumptions that are specified by the interface. Our methodology consists of three increasingly expressive classes of interfaces. Signature interfaces specify the methods that can be invoked by the user, together with parameters. Consistency interfaces add propositional constraints, enhancing signature interfaces with the ability to specify choice and causality. Protocol interfaces specify, in addition, temporal ordering constraints on method invocations. We provide approaches to check if two or more interfaces are compatible; if a web service can be safely substituted for another one; and if a web service satisfies a specification that represents a desired behavioral property.