Sustainability intersects Information and Communication Technology in two domains: Green IT (how can we make ICT itself more sustainable?) and Green by IT (how can we achieve sustainability through ICT?). On a closer look, it is software that links these two fields: In “classic” Green IT, there are many ways to build and use hardware in a more energy-efficient way. On the software side, Green by IT has often been software-based until now, involving tools that help to optimize logistics and automate processes to save energy, for example. However, the debate over software-induced energy consumption is just beginning. To date, few studies have been conducted about the energy saving potential of software itself. Therefore, it is important to investigate the meaning of sustainable software and sustainable software engineering. This chapter provides definitions of these concepts. In addition, it presents a reference model of sustainable software as well as its engineering. However, it provides only a short introduction of the model itself. The sub-model “Sustainability Criteria for Software Products” and sustainable software process models are examined in greater detail.