Software start-ups are new companies aiming to launch an innovative product to mass markets fast with minimal resources. However, most start-ups fail before realizing their potential. Poor software engineering, among other factors, could be a significant contributor to the challenges that start-ups experience. Little is known about the engineering context in start-up companies. On the surface, start-ups are characterized by uncertainty, high risk, and minimal resources. However, such a characterization isn't granular enough to support identification of specific engineering challenges and to devise start-up-specific engineering practices. The first step toward an understanding of software engineering in start-ups is the definition of a Start-Up Context Map - a taxonomy of engineering practices, environment factors, and goals influencing the engineering process. This map aims to support further research on the field and serve as an engineering decision support tool for start-ups. This article is part of a theme issue on Process Improvement.