Abraham Maslow posited that there are five stages of need: physiological, safety, social, self-esteem, and self-actualization. He stated that each need had to be met before people could progress to the level. Later, he added a sixth need, self-transcendance, which refers to the need to give to others.

Clayton Alderfer, a psychologist, amended Maslow's hierarchy of needs to produce ERG Theory, which refers to existence, relatedness, and growth. In Alderfer's hierarchy, there are only three levels instead of five. He posited that people can be trying to meet needs on more than one level at one time and that they can move from one level back and forth to another more fluidly than in Maslow's hierarchy of needs. If one's needs are not met at one level, they can regress to a lower level to have their needs met. This is the frustration-regression principle and can be applied to organizational analysis to think about why employees whose needs for growth are not met may decide to spend more time socializing to have their needs for relatedness met.  In addition, Alderfer stated that people's needs can change at different times of their lives, so that they are not static. 

Maslow’s hierarchy of needs and Alderfer’s ERG theories have several similarities and differences. Some of the similarities include;

1. Both the theories were formulated to explain human motivation in influencing behavior
2. Both theories have needs arranged in certain hierarchy, lower needs at the bottom and higher needs at the top

Some of the differences include;

1. In Maslow’s theory, needs have been classified and arranged into five levels whereas in ERG needs have been classified and arranged into three levels
2. In Maslow’s theory, needs are satisfied one at a time in a progression manner from lower to higher needs whereas in ERG several needs can be satisfied at one time despite of their level
3. Whereas the ERG theory asserts that the order of the needs is different for different people, according to Maslow, the order of the needs is universal
4. In the ERG model, frustration-regression is allowed whereby one can regress to lower level needs that may be easier to achieve if they are unsuccessful in attaining higher level goals. There is no such provision in Maslow’s theory since the needs are only achieved in a bottom to up trend

ERG, (existence needs, relationship needs, growth needs) is another way to look at a person’s psychological/sociological drives, a companion rather than a competitor to the more universally known five-step Maslow’s hierarchy of needs—physiological needs, security needs, social needs, esteem needs, self-actualization needs.  The similarities are that they both rise up from primitive survival drives to abstract, mental/sociological needs, and that they are met and satisfied in a certain order.  Alderfer’s division, an extension of Maslow’s original work, concerns itself primarily with motivation in the workplace—the value of rewards other than simple wages, the importance of titles, etc.  In both cases, the applied psychology begins to address what is meant by “human nature,” the universal needs of persons in all societies.