The proposed monograph, the second of the books “Typology of Deviant Development,” is devoted to the problem of insufficient development. The authors propose their own three-component model for the analysis of mental development and identify differential diagnostic criteria for distinguishing between variants and forms of insufficient development. The book provides detailed characteristics of all currently known categories of children with insufficient development. For each category, the characteristics of early childhood, neurological and somatic manifestations, the specifics of complex support and correctional work, the conditions that should be created for the child in accordance with his special educational needs are described, a developmental prognosis and the optimal educational route are given. The book is addressed to education specialists, teachers, psychologists, defectologists, speech therapists, social workers in general education and special (correctional) institutions, specialists in psychological and pedagogical support for children with developmental disabilities in the social protection and healthcare system. In addition, it will be of interest to neurologists, psychiatrists, pediatricians, and other health care workers.