Need theory is a concept that refers to the idea that humans have basic needs that must be met in order for them to function properly. Some common synonyms for need theory include motivational theory, requirements theory, and necessities theory. Another synonym is the hierarchy of needs theory, which outlines a set of needs that must be fulfilled in a specific order, beginning with basic physiological needs like food and shelter, and moving up to higher-level needs like love and esteem. Whatever term is used, the underlying idea is that people have a set of essential needs that must be met for them to lead healthy, fulfilling lives.