Primary care plays a crucial role in improving the overall quality of healthcare systems, as it serves as the first point of access to medical services in many countries. Therefore, ensuring accessibility and equity is essential. Primary care research largely falls within the field of health services research. Using Campbell’s conceptual model of access to primary care, this article highlights four major research domains: (1) identifying patient needs and converting them into demand, (2) evaluating accessibility, (3) assessing processes, and (4) evaluating outcomes. Representative studies are presented for each domain, illustrating how health services research in primary care provides evidence for policy development, enhances healthcare quality and equity, and strengthens overall service delivery.