In these essays, I empirically estimate the impact of interactions between primary care physicians (PCPs) and specialists on physician behavior. In the first chapter, I examine the impact that the acquisition of PCPs by firms employing specialists has on patient referral patterns in markets where integration takes place. In the second chapter, I estimate a model of physician entry behavior to understand the extent to which PCPs and specialists consider the number of physicians of the other type in the market when selecting which markets to enter.