Growth refers to an increase in size due to cell multiplication and addition of intracellular substances. Development is a broader term that refers to qualitative changes involving maturation of functions and acquisition of skills through experience that allows for improved functioning. Some key principles of growth and development include: 1) progression from head to toe, 2) progression from the center of the body outward, and 3) dependence on biological maturation and environmental learning. Development generally proceeds from simple to complex skills and is a continuous process building upon previous stages.