Genetic switches provide core components of gene expression induction systems, genetic circuits, and metabolite sensors, including those for high-throughput screening and selection of enzyme functions. However, it is rare that natural transcription factors meet the required specifications for each application. Fortunately, the directed evolution of transcription switches is a straightforward process, given that the two states of genetic switches (on and off) are both selectable, using a wide range of positive and negative selection tools developed in the field of molecular genetics. On/off-state selections based on bactericidal mechanism allow greatly accelerate the entire process. The key to success is finding the selection conditions that minimize false-positive and false-negative clones as with any directed evolution experiment. We introduce a reliable and automatable directed evolution platform to rapidly evolve genetic switches with desired specifications in this chapter. Highlighting the importance and ease of screening for selection conditions, we demonstrate how selection conditions influence the resultant population of the selected pools.