Synthetic models in biology are artifacts constructed from biological components, such as genes and proteins, to form biological parts or wholes. They have the same materiality as natural biological systems but are not the results of natural evolutionary processes. This entry focuses on synthetic genetic circuits, in particular the repressilator, which is one of the first and most famous of them. The introduction of the first synthetic genetic circuits in 2000 has often been taken as the beginning of synthetic biology proper. While they are intricate experimental systems, synthetic genetic circuits can be considered models in contrast to many other synthetic biology constructs. Being based on engineering principles and the enterprise of standardizing biological parts, synthetic models have enabled researchers to probe gene regulation by building small circuits implemented within living cells. Although synthetic modeling has been successful by many yardsticks, researchers have faced challenges due to biochemical particularities, as well as the stochasticity of biological systems and their unintuitive designs.