Component-based software development relies on reusable components in order to improve quality and flexibility of products as well as increasing development productivity. This paradigm promotes deployment of reusable components as black-box units that can only work and communicate with one another through their well defined interfaces. In this paper, understandability of component interfaces is considered as a major quality affecting reusability of software components. A set of metrics for measuring properties believed to be relevant to understandability and reusability of software components are presented. Then, their usefulness and relevance are analyzed based upon empirical data gathered from the measurement of a variety of component interfaces. The paper concludes with some ideas for further research in this area.