Software metrics programs are an important part of a software organization's productivity and quality initiatives as precursors to process-based improvement programs. Like other innovative practices, the implementation of metrics programs is prone to influences from the greater institutional environment the organization exists in. In this paper, we study the influence of both external and internal institutional forces on the assimilation of metrics programs in software organizations. We use previous case-based research in software metrics programs as well as prior work in institutional theory in proposing a model of metrics implementation. The theoretical model is tested on data collected through a survey from 214 metrics managers in defense-related and commercial software organizations. Our results show that external institutions, such as customers and competitors, and internal institutions, such as managers, directly influence the extent to which organizations change their internal work-processes around metrics programs. Additionally, the adaptation of work-processes leads to increased use of metrics programs in decision-making within the organization. Our research informs managers about the importance of management support and institutions in metrics programs adaptation. In addition, managers may note that the continued use of metrics information in decision-making is contingent on adapting the organization's work-processes around the metrics program. Without these investments in metrics program adaptation, the true business value in implementing metrics and software process improvement is not realized.