The theory of probability and statistics underlies all quantitative assessments relevant to pathology. Probability is a concept that occurs due to randomness. A random event or experiment has two main components of interest: the first is the “outcome” which is the result of the event that is being recorded. The second is “parameter” which is a constant in the experiment which can affect the outcome. Most events in biology are random and thus are governed by laws of probability, and, consequently, laboratory medicine where our concern is to measure these biological events is also governed by randomness and probability. Pathology and laboratory medicine employ the concepts of probability to provide us with a posttest probability of someone having a disease based on their test result, pretest probability, and the likelihood associated with the test. Another important concept is probability distribution which is the probability of occurrence of different possible outcomes of a random trial or experiment. Test characteristics such as “confidence interval,” “mean,” “reference range,” “error,” etc. are all determined using probability distribution.