The European commission recognises Cloud potential to improve competitiveness by enabling transformation to better connected and efficient society. However, still trust and security concerns hamper its massive adoption, both in private and public sectors. Towards a establishing a Trusted Cloud Europe this paper stresses the role that Service Level Agreements (SLAs) play by providing the mechanisms that allow both users and providers to establish a common understanding on the services to be provided and enforce guarantees around performance, transparency, conformance and data protection. To this end, the paper explores trust stakeholders and factors per cloud layer in the Cloud computing environment; it analyses the role of SLAs and provides a taxonomy of terms to support more tight and detailed SLA definitions that support users’ requirements in order to improve reliability and transparency in Cloud.