Empirical Software Engineering concerns the understanding of the practical processes of software product development in order to leverage the expertise and experience of the real people. We use a range of qualitative approaches, from in situ observation of professional practice to controlled studies, and a range of quantitative approaches, from data extraction from software repositories and its visualisation to modelling and simulation. Our approaches include multiple methods, triangulation, and longitudinal studies. Our emphasis is on practice in naturalistic settings, naturally-occurring artefacts, and reports from practitioners.