Needs-based motivation theories are based on the understanding that motivation stems from an individual's desire to fulfill or achieve a need. Human beings are motivated by unsatisfied needs, and certain lower needs must be satisfied before higher needs can be satisfied. In general terms, motivation can be defined as the desire to achieve a goal, combined with the energy, determination and opportunity to achieve it. This Wiki explores Abraham H. Maslow's Hierarchy of Needs theory, Clayton P. Alderfer's Existence Related Growth (ERG) Theory, and David McClelland's Need Theory, building off of work by Henry A. Murray.