This study addresses gaps in the research about the effect of professional development (PD) quality on effective instruction by taking a broad perspective when defining professional learning of teachers and by providing a theory-based explanation for effective instruction. Using a large-scale data set, Teaching and Learning International Survey (TALIS) 2013, the interrelationships between the quality of PD experience, professional communities at the school, and effective instruction were investigated. This theory testing study showed that the effect of PD events on effective instruction was stronger with the spillover effect of professional communities than with its sole effect.