The objective of this paper is to present the results of the study detailing the essence of the "gratefulness" phenomenon as a quintessence of morality in the context of today's world and its values. The methods of the research were theoretical analysis of the works of philosophers, psychologists, philologists, and pedagogues on the structure and essence of the notion "gratefulness" as well as the method of diagnostic experiment. A review of various treatments of gratefulness is suggested, ranging from the "feeling", "capacity", "attitude", "concept" to "indicator", "prerequisite", "limiter", "virtue", "duty", "obligation" and so on. An attempt is made to characterize "gratefulness" through description of its components and indices of its formation and a portrait of an adolescent having a high level of its formation is given. A complex of author's diagnostic techniques (in the forms of questionnaire, ranking, essay) worked out on the basis of the theoretical analysis and the current condition of gratefulness formation in adolescents from various types of schools is described. A comparative analysis of gratefulness formation in those schoolchildren who have development disorders and in normally developing ones is given.