The implementation of health information systems is known to be complex and challenging. To facilitate the introduction of IS in health care, research should investigate the way IS affects human actions and organizational structures and the reasons it affects them that way. Studying the dynamic relationship between cause and effect could help make better design and implementation decisions to achieve desired outcomes. Two theories are presented to help investigate the what and why of implementation. Structuration theory introduces the notion of the interdependency between human actions and organizational structures. Heeks theory of conception-reality gaps helps illuminate the causes of an implementation outcome. The two theories guide a case study of the implementation of health information systems in primary care clinics in Greece. The paper describes the two theories and how they may inform health care information systems research. It illustrates its points using examples from the field.