Cellulose is a component of all plant cells, and modification of the cellulose content or properties can have dramatic effects on the form and function(s) of specific parts or the entire plant. Cellulose synthase is the enzyme required for biosynthesis of cellulose, and a number of genes encoding this protein form part of a gene family in plants. Although plants are well endowed with genes for cellulose synthases, and expression of most of the CesA genes have been observed in most tissues, mutations in some of them can have very different effects. At the same time increased expression of some of the CesA genes may result in increased synthesis of cellulose in specific cells and tissues. More importantly, the direction in which the cellulose microfibrils are assembled in the primary cell wall helps determine the direction of cell elongation. In cells with a secondary cell wall, the orientation of the cellulose microfibrils influences the properties of the cell. Although the general view is that microtubules play a role in determining the direction of cellulose synthesis, not much is known as to how this occurs. For effective manipulation of cellulose synthesis in plant cells, it is necessary that we not only understand the machinery responsible for cellulose biosynthesis, but also as to how it is assembled, localized, and regulated.