利益冲突是指个人或组织涉及不同方面相同的利益时，向自己或与自己相关人士作出偏袒或优待的不当行为。利益冲突通常出现于公职人员或律师身上。如果没有一个完善的利益申报制度让公众监察，便很容易引起利益冲突出现。

通常，利益冲突是指专业服务领域特别是律师职业中的一种现象，即委托人的利益与提供专业服务的业者本人或者与其所代表的其他利益之间存在某种形式的对抗，进而有可能导致委托人的利益受损，或者有可能带来专业服务品质的实质性下降。专业服务过程当中存在利益冲突必将伤及专业服务的职业精神和特定职业的社会公信力。因此，无论立法或职业道德规范均要求专业服务业者或机构有义务采取有效措施避免出现利益冲突。通常的做法是，在接受一项委托前有义务进行利益冲突查证，确保专业服务业者或机构未向委托人的对立方（或者在利益上有对抗的任何一方）提供过或者正在提供服务；在接受委托后发现存在利益冲突的，专业服务业者或机构必须向委托人说明情况并主动辞去委托。在某些情形下，经过委托人的同意（通常要求书面形式），专业服务业者或机构可以豁免此项义务，比如：利益冲突并不显着的情形；专业服务业者或机构为与委托人有利益冲突的前委托人提供的服务已经超过了一定的年限等。

A conflict of interest (COI) is a situation in which a person or organization is involved in multiple interests, financial interest, or otherwise, one of which could possibly corrupt the motivation of the individual or organization.

The presence of a conflict of interest is independent of the occurrence of impropriety. Therefore, a conflict of interest can be discovered and voluntarily defused before any corruption occurs. A widely used definition is: "A conflict of interest is a set of circumstances that creates a risk that professional judgement or actions regarding a primary interest will be unduly influenced by a secondary interest." Primary interest refers to the principal goals of the profession or activity, such as the protection of clients, the health of patients, the integrity of research, and the duties of public office. Secondary interest includes not only financial gain but also such motives as the desire for professional advancement and the wish to do favours for family and friends, but conflict of interest rules usually focus on financial relationships because they are relatively more objective, fungible, and quantifiable. The secondary interests are not treated as wrong in themselves, but become objectionable when they are believed to have greater weight than the primary interests. The conflict in a conflict of interest exists whether or not a particular individual is actually influenced by the secondary interest. It exists if the circumstances are reasonably believed (on the basis of past experience and objective evidence) to create a risk that decisions may be unduly influenced by secondary interests.