Molecular switches are the fundamental building blocks in the field of synthetic biology. The majority of these switches is based on protein-protein, protein-DNA or protein-RNA interactions that are responsive towards endogenous metabolites or external stimuli like small molecules or light. By the rational and predictive reassembling of multiple compatible molecular switches, complex synthetic signaling networks can be engineered. Here we review how these switches were used for the regulation of important cellular processes at every level of the signaling cascade. In the second part we review how these switches can be assembled to open- and closed-loop control signaling networks and how these networks can be applied to facilitate cattle reproduction, to treat diabetes or to autonomously detect and cure disease states like gouty arthritis or cancer. © 2012 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.