Since one of the most common questions from the court to the forensic scientist pertains to the scientist's degree of certainity about the findings and conclusions at issue, scientists must consider how to determine probabilities. Major probability theories are the classical theory of probability, the apriori or logical theory of probability (AP), the relative frequency theory of probability (RF), and the subjectivistic theory of probability (SUB). Handwriting analysis mixes different types of judgments. These include findings and judgments from one specific case and judgments based on experience from a large number of previous cases. Theoretically, the overall judgment is covered by SUB and by AP. The frequency statements based on previous experience are covered by RF. There are still, however, a number of theoretical and practical difficulties in expressing judgments or degrees of belief in handwriting cases in terms of probabilities; therefore, the estimation of probabilities in handwriting cases should not be attempted except in special cases. The appendix details the characteristics of the various major probability theories. 3 references.