This chapter is the first of three consecutive chapters dedicated to data synthesis. We have split this topic into three chapters because the three different types of systematic review (quantitative, qualitative and mapping study) require very different procedures. Data synthesis is also one of the tasks that many software engineering researchers have identified as least well addressed by current guidelines, see Cruzes & Dybå (2011b) and Guzmán, Lampasona, Seaman & Rombach (2014).