Product metrics are objective measures of the structure of software artefacts. Specifically, product metrics can be used in at least three ways: making system-level predictions, early identification of high-risk software components, and the construction of preventative design & programming guidelines. These uses allow an organization to get an early estimate of software quality and to take early action to reduce the number of faulty software components. The objective of this report is to provide a review of contemporary object-oriented metrics. It first describes how object-oriented metrics can be used in practice by software organizations and presents an overview of some of the most popular object-oriented metrics & those that have been studied most extensively. The next section describes current cognitive theories used in software engineering that justify the development of object-oriented metrics. This is followed by a further elaboration of the cognitive theory to explain the cognitive mechanisms for metric thresholds. The empirical evidence supporting the above theories is then reviewed. The report concludes with recommendations for the practical usage of object-oriented metrics, a discussion of the match between the empirical results & theory, and directions for future research.