The development of web technologies and standards such as HTTP, XML, SOAP, WSDL, and UDDI enables pervasive adoption and deployment of web services. In a highly competitive business environment, quality of service is one of the substantial aspects for differentiating between similar service providers. A Service Level Agreement (SLA) between a service provider and its customers will assure customers that they can get the service they pay for and will obligate the service provider to achieve its service promises. Failing to meet SLAs could result in serious financial consequences for a provider. Hence, service providers are interested in gaining a good understanding of the relationship between what they can promise in an SLA and what their IT infrastructure is capable of delivering. Similarly, consumers are interested in understanding the impact of the SLAs they sign on their own productivity. In this paper, we present a novel approach to model and understand these relationships. Our model captures composition relationships between providers and consumers, as well the SLA between them. Our approach is based on simulation of the model and sensitivity analysis. 12 Pages