Psychomotor retardation refers to a situation when a child does not attain developmental milestones in the right time, i.e. he or she is unable to master skills that should develop at a given stage of life. Child development proceeds in strictly correlated phases. To attain a higher developmental level, one must first acquire and consolidate the lower level. The maturation of the central nervous system is quite intensive, even prenatally. Its individual structures develop in a specified order, depending on the week of gestation. Brain immaturity is a frequent cause of psychomotor retardation in children. The nervous tissue is composed of neurones (their cell membranes consist of a double lipid layer) and glial cells. Neurones form up to the 20th week of gestation, while glial cell formation starts after birth. This intensive development of the central nervous system may be accompanied by certain defects that will result in psychomotor retardation, which is also influenced by various physiological processes occurring in the child’s life. The type of damage and the grade of its severity depend on its location, area of pathological brain tissue and age at which the defect occurred. Injuries that cause psychomotor retardation in children may be divided into microinjuries, macroinjuries as well as endo- and exogenous causes. Psychomotor retardation is assessed on the Brunet–Lézine scale. It contains tasks that a child should be able to perform at a given stage of his or her development. The result is a psychomotor development quotient. Retardation may present as a complete deficit of given skills and their absence in the later stages of development. Psychomotor retardation may reveal itself at the beginning of the child’s life or in the later period. This depends on the causes of these limitations. The diagnosis of psychomotor retardation requires cooperation of various specialists.