Individuals who have studied maths to a specific level have come across Bayes’ Theorem or Bayes Formula. Bayes’ Theorem has many applications in areas such as mathematics, medicine, finance, engineering and many other. This paper covers Bayes’ Theorem at a basic level and looks at some of the history of this theorem as well as how the formula was derived. We also, look at some extended forms of the formula and give an explicit example. Lastly, we discuss recent applications of Bayes’ Theorem in valuating depression and predicting water quality conditions. We have chosen this topic for an exposition paper as it is an interesting topic in mathematics and our goal was to explain it in simple words for everyone to understand