Recent research links the development of a collaborative community of educators to enhanced teaching and learning effectiveness. This study contributes to this research by testing a work group effectiveness model with a sample of teachers from middle school teams. The study assesses the interrelationships among the model's antecedent variables (organizational context, design features, and interpersonal processes), intermediate variables (effort, knowledge/skills applied, and appropriateness of strategies), and final outcome variables (teaching and learning effectiveness). A survey developed to capture the concepts in the model was completed by 174 teacher members of interdisciplinary instructional teams in a large urban district's middle schools. Findings indicate that two fundamental variables, knowledge/skills applied to the work and appropriateness of strategies, are core mediators in the model. Two healthy interpersonal processes variables were also found to have direct effects on perceptions that teaming had improved teaching and learning.