The biology of romantic love has been explored by such biological sciences as evolutionary psychology, evolutionary biology, anthropology and neuroscience. Neurochemicals and hormones such as dopamine and oxytocin are studied along with a variety of interrelated brain systems which produce the psychological experience and behaviors of romantic love.

The study of romantic love is still in its infancy. As of 2021, there were a total of 42 biological studies on romantic love.

The meaning of the term "romantic love" has changed considerably throughout history, making it difficult to simply define. Initially it was coined to refer to certain attitudes and behaviors described in a body of literature now referred to as courtly love. However, academic psychology and especially biology also consider romantic love in a different sense, which refers to a brain system (or systems) related to pair bonding or mating with associated psychological properties.

Bode and Kushnick undertook a comprehensive review of romantic love from a biological perspective in 2021. They considered the psychology of romantic love, its mechanisms, development across the lifespan, functions, and evolutionary history. Based on the content of that review, they proposed a biological definition of romantic love:

Romantic love is a motivational state typically associated with a desire for long-term mating with a particular individual. It occurs across the lifespan and is associated with distinctive cognitive, emotional, behavioral, social, genetic, neural, and endocrine activity in both sexes. Throughout much of the life course, it serves mate choice, courtship, sex, and pair-bonding functions. It is a suite of adaptations and by-products that arose sometime during the recent evolutionary history of humans.

Romantic love in this sense is also not necessarily "dyadic", "social" or "interpersonal", despite being related to pair bonding. Romantic love can be experienced outside the context of a relationship, for example in the case of unrequited love where the feelings are not reciprocated. A person can develop romantic love feelings before any relationship has occurred, for only a potential partner. The potential partner can even be somebody they do not know well or aren't acquainted with at all, as in cases of love at first sight and parasocial attachments.

The early stage of romantic love (which has obsessive and addictive features) might also be referred to as being "in love", passionate love, infatuation, limerence or obsessive love. Research has never settled on a unified terminology or set of methods. Distinctions are drawn between this early stage of romantic love and the "attachment system" theorized by the attachment theorists like John Bowlby. In the past, attachment theorists have argued that attachment theory and attachment styles can replace other theories of love, but academics on love have argued this is incorrect and that romantic love and attachment are not identical concepts. The early stage of romantic love is thought to involve additional brain systems for other purposes, with distinct evolutionary histories. Romantic love is also distinct from sexual attraction, although they most often occur together.

Variation exists in the way romantic love is expressed in the population. A cross-cultural study of currently in-love people found four clusters, with varying degrees of intensity, obsessive thinking, commitment, frequency of sex and other differences. Other studies indicate romantic love can be experienced both with or without obsessional features. Typically, intense romantic love is limited to a duration of 12–18 months or as long as 3 years, depending on the estimate; however, in a rare phenomenon called "long-term intense romantic love", some people experience intense attraction inside a relationship, even for 10 years or more. This is similar to early-stage intense romantic love, but at this later stage they exhibit less of the obsessional features.