The Type A and Type B personality concept describes two contrasting personality types. In this hypothesis, personalities that are more competitive, highly organized, ambitious, goal-oriented, impatient, and highly aware of time management are labeled Type A, while more relaxed, "receptive", less "neurotic" and "frantic" personalities are labeled Type B.

The two cardiologists, Meyer Friedman and Ray Rosenman, who developed this theory came to believe that Type A personalities had a greater chance of developing coronary heart disease. Following the results of further studies and considerable controversy about the role of the tobacco industry funding of early research in this area, some reject, either partially or completely, the link between Type A personality and coronary disease. Nevertheless, this research had a significant effect on the development of the health psychology field, in which psychologists look at how an individual's mental state affects physical health.

Type A personality behavior was first described as a potential risk factor for heart disease in the 1950s by cardiologists Meyer Friedman and Ray Rosenman. They credit their insight to an upholsterer who called to their attention the peculiar fact that the chairs in their waiting rooms were worn out only on the arms and on the front edge of the seat. This suggested to Friedman and Rosenman that their patients were getting up from the chairs frequently and were otherwise waiting anxiously. After an eight-and-a-half-year-long study of healthy men between the ages of 35 and 59, Friedman and Rosenman estimated that Type A behavior more than doubled the risk of coronary heart disease in otherwise healthy individuals. The individuals enrolled in this study were followed well beyond the original time frame of the study. Participants were asked to fill out a questionnaire, that asked questions like "Do you feel guilty if you use spare time to relax?" and "Do you generally move, walk, and eat rapidly?" Subsequent analysis indicated that although Type A personality is associated with the incidence of coronary heart disease, it does not seem to be a risk factor for mortality. It was originally called 'Type A Personality' by Friedman and Roseman who defined it as "an action-emotion complex that can be observed in any person who is aggressively involved in a chronic incessant struggle to achieve more and more in less and less time, and if required to do so, against the opposing efforts of other things or other persons." It has now been conceptualized as the Type A behavior pattern.

Some contemporary psychologists argue that the Type A and Type B personality concept is outdated and oversimplified. A 1997 study found that Type A behavior, described by traits such as competitiveness and time urgency, could be correlated with higher scores for neuroticism and extraversion in the Five-Factor Model of personality. This suggests a more nuanced understanding of personality traits is necessary.

The hypothesis describes Type A individuals as outgoing, ambitious, rigidly organized, highly status-conscious, impatient, anxious, proactive, and concerned with time management. People with Type A personalities are often high-achieving workaholics. They push themselves with deadlines, and hate both delays and ambivalence. People with Type A personalities experience more job-related stress and less job satisfaction. They tend to set high expectations for themselves, and may believe others have these same high expectations of them as well. Interestingly, those with Type A personalities do not always outperform those with Type B personalities. Depending on the task and the individual's sense of time urgency and control, it can lead to poor results when there are complex decisions to be made. However, research has shown that Type A individuals are in general associated with higher performance and productivity. Moreover, Type A students tend to earn higher grades than Type B students, and Type-A faculty members were shown to be more productive than their Type B behavior counterparts (Taylor, Locke, Lee, & Gist, 1984).

In his 1996 book, Type A Behavior: Its Diagnosis and Treatment, Friedman suggests that dangerous Type A behavior is expressed through three major symptoms: (1) free-floating hostility, which can be triggered by even minor incidents; (2) time urgency and impatience, which causes irritation and exasperation usually described as being "short-fused"; and (3) a competitive drive, which causes stress and an achievement-driven mentality. The first of these symptoms is believed to be covert and therefore less observable, while the other two are more overt.

Type A people were said to be hasty, impatient, impulsive, hyperalert, potentially hostile, and angry. Research has also shown that Type A personalities may use certain defenses or ways of dealing with reality to avoid difficult realizations.^{[clarification needed]} For example, one study found that those with Type A personality are more likely to show higher levels of denial than Type B in stressful situations.

There are two main methods to assessing Type A behavior, the first being a structured interview (SI) developed by Friedman and Rosenman, and the second being the Jenkins Activity Survey (JAS). The SI assessment involves an interviewer's measuring a person's emotional, nonverbal, and verbal responses (expressive style). The JAS involves a self-questionnaire with three main categories: Speed and Impatience, Job Involvement, and Hard-Driving Competitiveness.