In social psychology, an interpersonal relation (or interpersonal relationship) describes a social association, connection, or affiliation between two or more people. It overlaps significantly with the concept of social relations, which are the fundamental unit of analysis within the social sciences. Relations vary in degrees of intimacy, self-disclosure, duration, reciprocity, and power distribution. The main themes or trends of the interpersonal relations are: family, kinship, friendship, love, marriage, business, employment, clubs, neighborhoods, ethical values, support, and solidarity. Interpersonal relations may be regulated by law, custom, or mutual agreement, and form the basis of social groups and societies. They appear when people communicate or act with each other within specific social contexts, and they thrive on equitable and reciprocal compromises.

Interdisciplinary analysis of relationships draws heavily upon the other social sciences, including, but not limited to: anthropology, communication, cultural studies, economics, linguistics, mathematics, political science, social work, and sociology. This scientific analysis had evolved during the 1990s and has become "relationship science", through the research done by Ellen Berscheid and Elaine Hatfield. This interdisciplinary science attempts to provide evidence-based conclusions through the use of data analysis.

Friendships are non-sexual, non-romantic interpersonal relationships. Social relations can be homosocial such as female bonding and male bonding or heterosocial such as cross-sex friendship. In some countries a reduction of friendships is observed, also called friendship recession or loneliness epidemic.

An intimate relationship is an interpersonal relationship that involves emotional or physical closeness between people and may include sexual intimacy and feelings of romance or love. Intimate relationships are interdependent, and the members of the relationship mutually influence each other. The quality and nature of the relationship depends on the interactions between individuals, and is derived from the unique context and history that builds between people over time.

Romantic relationships have been defined in countless ways, by writers, philosophers, religions, scientists, and in the modern day, relationship counselors. Two popular definitions of love are Sternberg's Triangular Theory of Love and Fisher's theory of love.^{[additional citation(s) needed]} Sternberg defines love in terms of intimacy, passion, and commitment, which he claims exist in varying levels in different romantic relationships. Fisher defines love as composed of three stages: attraction, romantic love, and attachment.

Romantic relationships can have great social and cultural variability, and may for example exist without any sexual intimacy, between people of any gender, or among a group of people, as in polyamory or open relationships.

While many individuals recognize the single defining quality of a romantic relationship as the presence of love, it is impossible for romantic relationships to survive without the component of interpersonal communication. Within romantic relationships, love is therefore equally difficult to define. Hazan and Shaver define love, using Ainsworth's attachment theory, as comprising proximity, emotional support, self-exploration, and separation distress when parted from the loved one. Other components commonly associated with love are physical attraction, similarity, reciprocity, and self-disclosure.

Early adolescent relationships are characterized by companionship, reciprocity, and sexual experiences. As emerging adults mature, they begin to develop attachment and caring qualities in their relationships, including love, bonding, security, and support for partners. Earlier relationships also tend to be shorter and exhibit greater involvement with social networks. Later relationships are often marked by shrinking social networks, as the couple dedicates more time to each other than to associates. Later relationships also tend to exhibit higher levels of commitment.