Until now, the quality problems of quantum software have been largely ignored. This chapter analyzes the applicability of models and metrics for quantum software and, to mitigate this lack of attention to quality issues, presents a set of metrics that have been proposed and empirically validated to characterize the complexity of quantum circuits in terms of their understandability. The validation experiment design, execution, and results are reported. In addition, the main functionalities of a prototype tool that has been created for the automatic computation of the metrics are briefly presented.