We can all tell when a movie of some everyday event,such as a kettle boiling or a glass shattering,is run backwards.Similarly,we all feel that we can remember the past and affect the future,not vice versa.So there is a very clear "arrow" (direction) of time built into our interpretation of our everyday experience.Yet the fundamental microscopic laws of physics,be they classical or quantum-mechanical,look exactly the same if the direction of time is reversed.So what is the origin of the "arrow" of time? This is one of the deepest questions in physics;I will review some relevant considerations,but do not pretend to give a complete answer.