Written as a comprehensive review of the topic, the author introduces the reader to the concept of nondeterministic and probabilistic models and how nondeterminism can be resolved. The author then considers probabilistic models, three anomalies of these models and how to overcome them. The author then addresses alternative characterizations of the two probabilistic trace equivalences. Finally, the author considers the anomalies of probabilistic testing equivalence and how to avoid them. This monograph is aimed at researchers working on the formal method aspects of programming languages.