The book sets out historically established ideas in science about the delay in mental development in children, about the features of their cognitive activity, personality and communication. Much attention is paid to the study of thinking and speech development in preschool and primary school age. The results of the author's research on the main causes of difficulties in teaching writing, reading, mathematics, manifested in the educational process, are presented. The book is intended for students studying in the specialty "Clinical Psychology." The content of the textbook can attract the attention of specialists in the field of inclusive education: psychologists, speech therapists, defectologists, social educators.