Although the understanding on the theories of crystal growth is not an absolute requirement to obtain single crystals, the techniques can be better applied if basic aspects of the theories are known. This chapter begins with a discussion on crystal morphology, starting with a question of why single crystals of perovskite compounds have a cubic shape, spinel compounds an octahedral shape, and garnet compounds a dodecahedral shape. The chapter then moves on to the mechanisms of flux growth. The concepts of solubility and supersaturation are introduced, and the three main steps of crystal growth—the attainment of supersaturation, the formation of nuclei of the crystalline phase, and subsequent crystal growth on the nuclei—are described. The discussion gives the answer to why some crystals have flat faces whereas others show hopper or dendritic morphologies. The chapter ends with descriptions on various imperfections sometimes found in flux-grown crystals.