Maslow's theory of motivation proposes that there is a hierarchy of five basic human needs: physiological needs, safety needs, social needs, esteem needs, and self-actualization needs. Physiological needs are the most basic needs, including food, water, shelter, and sex. Once these are met, the next level of safety needs become salient, followed by social needs, esteem needs, and ultimately the need for self-actualization at the top level. The theory suggests that people are motivated to fulfill lower level basic needs before progressing on to meet higher level growth needs.