This chapter introduces falsificationism - a philosophy of science based on the notion that science is based on critical testing of hypotheses about how the world is. A short piece of fiction is first used to illustrate this idea. Building on this story, hypothesis-testing is introduced as a general method, and in particular Karl Popper’s falsificationism. Popper’s claim that logic supports falsification rather than generalisation is introduced. It is outlined how falsificationism may be superior to at least naive empiricism as a philosophy of science. It provides a shift in perspective; theory now precedes observations rather than the opposite. The concept of falsifiability and degrees of falsifiability (how falsificationism rewards clarity and precision) are introduced. The role of predictions, and how hypothesis-testing rather than empiricism addresses causality and explains change, is discussed, as well as why ad hoc hypotheses that are not independently testable should be avoided according to Popper.