Revision with unchanged content. The understanding and the ability to manipulate physical systems as small as clusters have become a more and more urgent demand as technology is approaching this size range. Magnetism of clusters is especially intriguing not only because of its great application potential but also because it provides new insight to bulk magnetism which is still not fully understood. This book presents an extensive study of the magnetic properties of cobalt clusters using molecular beam method. It is shown that the magnetization of ferromagnetic clusters in a cluster beam can be understood as an adiabatic process using the avoided crossing theory. Two states with quantized magnetic moment per atom have been discovered for both Co and Fe clusters. This may explain the fractional magnetic moments as well as the local magnetic order observed above the Curie temperatures for iron group ferromagnets. This book provides a firm ground for measuring magnetic moment using Stern-Gerlach experiment as well as new understanding of magnetism. It should be helpful for anyone who is interested in understanding or research in magnetism.