This book investigates the chemistry and physics of complex systems by focusing on the role of spatial restrictions on molecular movement. It covers a broad range of topics including motions of excitations, multilayers and porous glasses, defect diffusion in polymers, photoreactions, nuclear magnetic resonance, and orientational dynamics. Methods are critically evaluated as they are used, and the book concentrates on examples that revolve around the use of probes. It utilizes a classical approach as well as modern fractal ideas of spatial relations. The text will benefit biochemists, chemists, physicists, and graduate students of chemistry and physics.