One part of the paper is devoted to frequency interpretation, as advocated by early K. Popper. However, the main subject of the paper is Popper’s own propensity interpretation of probability. It outlines the fundamental principles of the propensity interpretation, as well as its relation to quantum theory and its various variants. In conclusion it examines the so called Humpreys’s paradox as related to the interpretation of causality in the frame of Popper’s propensity interpretation.