The probability of an event refers to the likelihood that the event will occur. Nearly every day you use probability to plan around the weather. Meteorologists can’t predict exactly what the weather will be, so they use tools and instruments to determine the likelihood that it will rain, snow or hail. This book provides an introduction to probability that is both mathematical, in the sense that the underlying theory is developed from axioms, and practical, in the sense that applications to real-world problems are discussed. It is essential that we grasp the difference between theory and reality. Theories are ideas proposed to explain phenomena in the real world. As such they are approximations or models of reality.