Interest in the physics of complex macroscopic systems is growing rapidly. Such systems, composed of several subsystems interacting in a non linear way, display fascinating and sometimes surprising phenomena such as self-organization, sharp transitions among qualitatively different states, noise-induced phenomena and seemingly chaotic dynamics. This book provides a wide and unitary approach to the field. which is achieved by stressing the role of the reduction procedures, i.e. of the passage from more detailed to more synthetic dynamical descriptions. The book is aimed at university level readership although emphasis is placed on physical intuition rather than mathematical details. To this end, theoretical sections outlining the major concepts and mathematical techniques needed for the analysis of complex systems are integrated by relevant applications to physical and chemical systems.