This chapter is about how to build applications in the cloud. It cannot cover this topic exhaustively. Not only is there a wide range of aspects involved, such as analysis, design, implementation, testing and operations, but there are also several different architectural approaches that can be used, each of which could easily fill not just a chapter, but a whole book. Therefore, the goal of this chapter is to give an overview of some of the most important architectural approaches, and to provide some guidance on their relative strengths and weaknesses. Additionally, several aspects that apply across these different approaches are covered.