How do unicellular organisms find food and avoid noxious substances? How do white blood cells identify and approach invading microorganisms? How are sperm cells directed to the egg? The common answer to all these questions is: by chemotaxis. Chemotaxis - the response of cells to chemical stimuli by directed movement - is one of the most basic processes in nature. In this text, the author demonstrates how basic chemotaxis is to life, and how widespread it is, and presents the state of the art with respect to its molecular and physiological mechanisms. Because chemotaxis research is still in progress, the book also indicates open questions and future directions of research. In addition, potential applications to health problems are pointed out.