Artificially engineered ribozymes can be used to specifically regulate expression of target genes. Such ribozymes can be synthesized chemically and delivered into the cell exogeneously. Alternatively, ribozymes can be produced by the cell endogenously, after introduction of the artificial gene into the cellular genome. In the latter case, the design of the artificial gene defines the ribozyme properties, such as: expression level, intracellular localization, folding and association with proteins. Generally speaking, design of the expression vector is critical to obtain active ribozyme molecules. This paper first describes factors that are known or predicted to affect ribozyme activity in the cell, then reviews various expression systems that have been specifically developed for ribozymes. Lastly, a recently developed ribozyme system termed snorbozymes (small nucleolar RNA:ribozyme hybrids) will be discussed. This powerful test system has generated several important observations that are likely to affect the future development of ribozyme technology.