There is experimental evidence that the direction of social, intellectual, physical, personality, and emotional development are determined more by events at certain stages than it is earlier or later. These stages have been called “critical periods. ” This artiecle reviews the evidence and interprets the experimental results for a general theory. Although most of the work reported here makes use of animal subjected, the main concepts can be applied, at least catiously to human development. The “critical period” concept can be considered to have four major components (Gordon Pryor, 1975). 1) “Critical periods” are usually assumed to occur prenatally or very soon after birth. This main emphasis is usually placed on early development when many changes are occurring at their most rapid rate. 2) “Critical periods” are assumed to occur during a relatively short time period in the developmental sequence-perhaps only a few hours, days, or at most weeks or months, depending upon the species and its total life span. 3) Effective endogenous or exogenous stimulation that occurs during a “critical period” is assumed to have long-lasting consequences on subsequent development. 4) Such stimulation is assumed to have little or no effect on the parameters in question when it precedes or follows a “critical period.” Freud had suggested the importance of early experience and pointed toward the existance of critical periods in human personality development. Though many experiments, it is clear that perinatal dietary restiction can have rather severe and Ion-lasting effects on subsequent development especially in brain. Some young birds-ducklings and chicks become attached to any object to which they are long exposed during the critical period, even when their contact is only visual. The barren environments of all sorts, including large and impersonal infant hospital and traditional orphanages for young children are emotionally and intellectually severely damaging to the child, and blunt the development not only of the human organism but of simpler nonverbal organisms such as rats, mice, dogs, and monkeys. The concept of critical periods is highly important for human and animal welfare. The dangers and potential benefits for each period of life are known, it should be possible to avoid the former and take advantage of the latter. The discovery of critical periods immediately focuses attention on the developmental processes which cause them. As these processes become understood, it is increasingly possible to deliberately modify critical periods and their results.