Abstract In scientific practice, simplicity is a valued quality of theories. When scientists have a choice between possible explanations, they usually prefer the simplest. In the same vein, Ockham's razor is a cherished methodological rule. It urges us not to multiply conceptual and ontological entities beyond necessity in our explanations. But why should we prefer simplicity? Is there an epistemological reason to do so? And what is meant by simplicity? This book argues that the approach of Solomonoff subsumes most of the other approaches and ideas, and solves most of their problems. I further demonstrate that searching for simplicity also serves several other goals of science, i.e., generalisation, unification, reduction, idealisation and concretisation. http://simplicity-in-science.org