In recent years, medical professionals are in charge with multiple roles. They have to work as an educator, researcher, and administrator, as well as medical practitioner. In addition, they experience a conflict between the primary responsibilities that each role requires of them. A conflict of interest (COI) is a set of circumstances that creates a risk that professional judgment or actions regarding a primary interest will be unduly influenced by a secondary interest. It occurs when an individual or organization is involved in multiple interests, one of which could possibly corrupt the motivation for an act in the other. The COI should be managed appropriately to preserve the value of public trust, scientific objectivity, and the benefit and safety of patients. Primary interest of medical professionals refers to the principal goals of the medical profession, such as the health and safety of patients, and the integrity of research. Secondary interest includes not only financial gain but also such motives as the desire for professional advancement and the wish to do favors for family and friends, but COI rules usually focus on financial relationships because they are relatively more objective, fungible, and quantifiable. This article will briefly review the COI in medical practice and research, discuss about what is COI, why we should manage it, and how we can manage it.