RNA and DNA molecules can be engineered to perform as precision allosteric enzymes, or "molecular switches", that are modulated by specific effectors. These designer sensor elements have numerous applications ranging from the construction of biosensors to the development of novel genetic switches. We have embarked on a program to establish effective molecular engineering strategies for switch construction, and to establish the fundamental principles that dictate the performance characteristics of these molecules. In pursuing our objectives, we have created a variety of RNA molecular switches that are modulated by specific target molecules that range from nucleotides and oligonucleotides to drug compounds, metabolites and toxins. In addition, we have begun to explore the use of immobilized RNA switches for the construction of advanced biosensor arrays. Our findings suggest that RNA and DNA have a significant untapped potential for functioning as precision molecular switches in both industrial and natural settings.