Cellulosic materials are the major components of fibrous biomass produced as a result of photosynthesis and are considered as a reservoir of solar energy and organic materials. In order to cope with the problems of food and energy shortages expected in the near future, biotechnologists are encouraged to develop new technologies for the more effective utilization of the world's sustainable resources, i.e., biomass. One way is to engineer microorganisms and animals with the capability of digesting and utilizing cellulosic materials, and plants which can be easily degraded by cellulolytic enzymes. In this article, we summarize recent studies on the molecular breeding of cellulolytic organisms for biomass utilization along with some considerations regarding cellulolytic enzymes.