In psychology, as well as other social and behavioral sciences, aggression (also called combativeness) refers to behavior between members of the same species that is intended to cause pain or harm. Predatory behavior between members of one species towards another species is also described as "aggression." To exhibit aggression towards members of another species is common, such as in these examples: "Lions are aggressive hunters of antelopes," and "Eagles are aggressive hunters of small mammals."

Aggression takes a variety of forms among human beings, and it can be physical, mental, or verbal. Aggression should not be confused with assertiveness, although the terms are often used interchangeably among laypeople, e.g. an aggressive salesperson.

There are two broad categories of aggression. These include hostile, affective, or retaliatory aggression and instrumental, predatory, or goal-oriented aggression. Empirical research indicates that there is a critical difference between the two, both psychologically and physiologically. Some research indicates that people with tendencies toward "affective" aggression, defined in this study as being "impulsive, unplanned, overt, or uncontrolled" have lower IQs than those with tendencies toward "predatory" aggression, defined here as being "goal-oriented, planned, hidden, or controlled".

Across many different human cultures, men are more likely than women to express aggression by means of direct physical violence. Women are more likely to instead express aggression through a variety of indirect or nonphysical means.