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The integration of cloud and grid infrastructures is still of current interest, because it provides a way for the scientific area to ensure sustainability of well engineered grid applications. The integration of well established grid infrastructures with cloud systems also fosters their complementary usage, simplified migration of applications, as well as efficient resource utilization. In this paper, we compare the layered conceptual grid model to the service model of clouds. Based on this comparison, we describe pragmatic possibilities to integrate cloud and grid systems. We analyze the connectivity options on the infrastructure level to gain access to both infrastructures using a unified client. In two case studies, we show the successful integration of the Amazon Web Services cloud with UNICORE~6 and the open source cloud Eucalyptus with Globus Toolkit~4. Based on these implementations, we discuss lessons learned.