Bayes theorem is a widely used relationship in statistics and machine learning. It is used to find the conditional probability of an event occurring, ie. the probability that the event will occur given that another (related) event has occurred. It is named after the Reverend Thomas Bayes, an English statistician and Presbyterian minister, who formulated Bayes theorem in the 1700's. It has become a popular method for calculating conditional probabilities in many real-world situations.