Microcrystals large enough to be seen in an electron microscope have long been objects of intense scientific and technological interest. Recently, it has become possible to study properties of even smaller units of matter, clusters, thus allowing the last gap betweeen molecules and crystals to be bridged. This book presents the experimental and theoretical techniques needed to study this new state of condensed matter. The material is presented so as to be understandable to an audience with varied backgrounds and interests. At the beginning of each part basic concepts are quickly reviewed for graduate students and for experts who wish to broaden their interest in theory and experiments. The concepts are then applied to current problems of general interest. The basic subjects covered by the book are cluster sources, the evolution of electronic properties of condensed matter, the microscopic view of crystal growth, the chemical reactivity of clusters, cluster stability and fragmentation.