The book is a comprehensive introduction to the work of the Estonian-German biologist Jakob von Uexk ll. After a first introductory chapter by Morten T nnessen and a second chapter on Uexk ll's life and philosophical background, it contains four chapters devoted to the analysis of his main works. They are followed by a vast eighth chapter which deals with the influence Uexk ll had on other philosophers and scientists. Finally, the author discloses his conclusions, focused on the possibility of updating Uexk ll's work. As far as the key issue is concerned, the Uexk llian Umwelt is the perceptive and operative world which surrounds animal species; it is a subjective species-specific construction which provides living organisms with great security and behaviour stability. The relationship that the animal carries out with its environment is a complex system of semiotic interactions: its behaviour is not a set of mechanical reactions, but a spontaneous attribution of meaning to the outside world.