Despite the advantages and rapid growth of Cloud computing, the cloud environments are still not sufficiently trustworthy from a customer’s perspective. Several challenges such as specification of service level agreements, standards, security measures, selection of service providers and computation of trust still persists, that concerns the customer. To deal with these challenges and provide a trustworthy environment, a mediation layer may be essential. In this paper we propose a cloud broker as a mediation layer, to deal with complex decision of selecting a trustworthy cloud provider, that fulfils the service requirements, create agreements and also provisions security. The cloud broker operates in different modes and this enables a variety of trust assessments.