The main purpose of this book is to describe ways of assessing forensic science evidence and the means of communicating this assessment to a court of law. A clear exposition of probability from the Bayesian perspective is provided. The underlying theme of the book is the emphasis on the importance, for the assessment of the value of associative evidence linking a suspect and a crime scene, or the comparison of two probabilities, the first being that of the evidence if the suspect is guilty, the second being that of the evidence if the suspect is innocent.