Cellular growth, especially its pattern formation, has been studied both experimentally and numerically. In situ observations of faceted cellular growth have clearly revealed cellular interactions in the array of cells. For the first time, the true time-dependent faceted cellular array growth has been modelled properly. It has been found that pattern formation is determined by cellular interactions in the array. Readers of the book will obtain a general view of the field of pattern formation in crystal growth, and in-depth and up-to-date knowledge of faceted cellular array growth, which occurs in semiconductor crystals.