Defining the concept of infidelity in relationships has posed significant challenges for many researchers. By synthesizing their views, one can assume that infidelity encompasses any actions hidden from a partner, involving a third party, which violate the emotional, sexual, or romantic exclusivity of the relationship. Nowadays, it seems appropriate to categorize infidelity into sexual, emotional, mixed, and online types. Determining the frequency of infidelity is problematic due to discrepancies in various studies’ results, primarily stemming from cultural differences among the studied populations. Nonetheless, it is expected that this phenomenon affects 20-50% of relationships, occurring more frequently in non-marital relationships, although this difference appears to be gradually diminishing. The causes of infidelity are diverse, primarily arising from the unmet emotional or sexual needs of an individual by their partner. However, infidelity can also occur due to factors beyond the partner’s control, such as favorable circumstances. Factors that increase the likelihood of infidelity include being male and having higher education and income, as these factors lead to encountering more situations that facilitate infidelity. The impact of technological development on the occurrence of infidelity in relationships cannot be overlooked – practically every third internet user utilizes it for sexual purposes. Infidelity has consequences for relationships, often being the most common cause of their dissolution, as well as for the individuals involved. Those who are betrayed often experience anger and symptoms typical of a depressive episode and suicidal thoughts may also occur.