The book contains an analytical review of more than 800 studies of assertive behavior and the results of the author's own research. The qualities of assertive individuals, the impact on assertiveness of age, gender, family, situational, ethnic and cultural factors. The advantages of assertiveness, the perception of assertive behavior and the expectation of its consequences, features of assertiveness in childhood and the influence of parents and school on the development of assertiveness in children, the connection between the emotional intelligence of the child and his assertiveness are revealed. The positive connection of assertiveness with psychological health and negative - with depression and addictions dangerous for health and life is shown. Assertiveness in service relations, its role in teamwork and leadership is studied. The methods of teaching assertiveness are described. A reliable and valid assertiveness test is offered.