Annulment is a legal procedure within secular and religious legal systems for declaring a marriage null and void. Unlike divorce, it is usually retroactive, meaning that an annulled marriage is considered to be invalid from the beginning almost as if it had never taken place (though some jurisdictions provide that the marriage is only void from the date of the annulment.

Although an annulment is thus a declaration that "the marriage never existed", the Church recognizes that the relationship was a putative marriage, which gives rise to "natural obligations". In canon law, children conceived or born of either a valid or a putative marriage are considered legitimate and illegitimate children are legitimized by a putative marriage of their parents, as by a valid marriage. Certain conditions are necessary for the marriage contract to be valid in canon law. Lack of any of these conditions makes a marriage invalid and constitutes legal grounds for a declaration of nullity. Accordingly, apart from the question of diriment impediments dealt with below, there is a fourfold classification of contractual defects: defect of form, defect of contract, defect of willingness, defect of capacity. For annulment, proof is required of the existence of one of these defects, since canon law presumes all marriages are valid until proven otherwise.

A diriment impediment prevents a marriage from being validly contracted at all and renders the union a putative marriage, while a prohibitory impediment renders a marriage valid but not licit. The union resulting is called a putative marriage. An invalid marriage may be subsequently convalidated, either by simple convalidation renewal of consent that replaces invalid consent.