Personality psychology concerns the nature of human nature and tells us how a person will act in different situations and why. This book tells the story about the differences and similarities between people, and the causes and consequences of these differences. It commences with a note on the salient psychological theories of personality. During the mid-20th century, behaviorism emerged as a dominant paradigm for understanding human behavior, including personality. Although the social cognitive theory of personality has its origins in the radical behaviorist tradition, it emerged in clear opposition to it. Causal theories of personality deal with the question of why people differ in various ways. Behavioral genetics, an area of psychology concerned with the assessment of the relative contribution of genetic and nongenetic influences on various individual variables of difference, including personality, intelligence, and psychological disorders, is also outlined. Psychologists believe people can measure personality using reliable scientific tools. There has been an increased interest in alternative methods for objectively assessing personality. One compelling example is the Implicit Association Test (IAT). The book also shows how personality influences what is traditionally seen as social and cultural phenomena, such as political attitudes and religious beliefs, and prosocial and antisocial behavior. According to research, the most important personality correlates of prosocial behavior are extraversion and agreeableness. The book concludes with a note on the implications of using personality inventories in the context of identifying bad or problematic traits, such as narcissism, Machiavellianism, and psychopathy, and online personality profiling in the context of consumer behavior.