In this era of high-stakes testing and tight funding there is unprecedented interest in and a requirement for accountability in the field of education. Virtually all funded projects are required to have an evaluation component designed to determine if project goals have been met. Positive outcomes are often the basis for continued funding and implementation. School systems also depend heavily on well-designed evaluations to assess the quality and impact of the professional development they offer to bring about change in teacher practice, in their effort to implement reform, and to demonstrate accountability to their stakeholders. The need to provide and assess professional development to improve teaching practices has generated numerous evaluation models that are widely used but have not been empirically tested. Since important program decisions are based on the results of these assessments, there is a great need to ensure the efficacy of these evaluation models to appropriately assess the programs they are intended to evaluate. Therefore, the purpose of this research was to empirically test the theory underlying Guskey\u27s Model for evaluating professional development, which is widely used by school systems engaging in program assessment. This study focused on testing the nomological network of one of the most commonly used evaluation models developed by Thomas Guskey. A description of the model is presented along with a discussion of the lack of empirical evidence that exists regarding its effectiveness. By investigating the relationships among the five components in Guskey\u27s Model (Teacher Satisfaction, Teacher Knowledge, Teacher Practices, Administrative Support and Student Outcomes), it was possible to determine whether these assumed relationships actually do exist and contribute to the accuracy of the program evaluation. Data collected from Reading First Ohio over the past 6 years was utilized to test the nomological net of Guskey\u27s model. The finding indicated strong support for the continued used of Guske