E-commerce is a new form of trade that takes increasingly importance in various fields, such as tourism or transport. With the spread of the Internet, users become more demanding and want to express and get answers to more complex requests, including multiple functionalities, conditions, constraints and objectives. Most complex requests including multiple functionalities cannot usually be answered by one single Web service. As multiple services are needed, the problem is then to find good combinations using the available services but also to select the best ones according to user objectives expressed in the request. This book contributes to resolving this issue by focusing on the problem of semantic Web service composition and optimization to answer such requests. For the automatic design of service composition satisfying a request, we propose a new model for the representation of semantic Web service composition and an algorithm that builds compositions based on this model. This latter supports any kind of composition structure. To select the best compositions with a suitable method, the problem of composition optimization is defined according to the request complexity.