Gratitude, appreciation, or thankfulness is a positive emotion or attitude in acknowledgment of a benefit that one has received or will receive.

In a religious context, gratitude can also refer to a feeling of indebtedness towards a deity. Most religions prescribe rituals of thanksgiving towards their higher powers; the expression of gratitude to God is a central theme of Christianity and Islam.

In contrast to the positive feeling of gratitude, the feeling of indebtedness is a negative reaction to a favor (Tsang, 2006a; Watkins, Scheer, Ovnicek, & Kolts, 2006). Even though our reactions to favors might not always be positive, researchers have found that people express gratitude often. In a 1998 Gallup poll, the majority of Americans said they express gratitude to God (54%) and others (67%) "all the time."

Psychological research has demonstrated that individuals are more likely to experience gratitude when they receive a favor that is perceived to be (1) valued by the recipient, (2) costly to the benefactor, (3) given by the benefactor with benevolent intentions, and (4) given gratuitously (rather than out of role-based obligations) (e.g., Bar-Tal, Bar-Zohar, Greenberg, & Hermon, 1977; Graham, 1988; Lane & Anderson, 1976; Tesser, Gatewood, & Driver, 1968). Individuals who are induced to feel grateful are more likely to behave prosocially toward their benefactor (Tsang, 2006b) or toward unrelated others (Bartlett & DeSteno, 2006).

Gratitude may also serve to reinforce future prosocial behavior in benefactors. For example, Carey and colleagues (Carey, Clicque, Leighton, & Milton, 1976) found that customers of a jewelry store who were called and thanked showed a subsequent 70% increase in purchases. In comparison, customers who were thanked and told about a sale showed only a 30% increase in purchases, and customers who were not called at all did not show an increase. Rind and Bordia (1995) found that restaurant patrons gave bigger tips when their servers wrote “Thank you” on their checks.

Research has also suggested that feelings of gratitude may be beneficial to subjective emotional well-being (Emmons & McCullough, 2003). For example, Watkins and colleagues (Watkins et al., 2003) had participants test a number of different gratitude exercises, such as thinking about a living person for whom they were grateful, writing about someone for whom they were grateful, and writing a letter to deliver to someone for whom they were grateful. Participants in the control condition were asked to describe their living room. Participant who engaged in a gratitude exercise showed increases in their experiences of positive emotion immediately after the exercise, and this effect was strongest for participants who were asked to think about a person for whom they were grateful. Participants who had grateful personalities to begin with showed the greatest benefit from these gratitude exercises. In people who are grateful in general, life events have little influence on experienced gratitude (McCullough, Tsang & Emmons, 2004).