Synthetic biologists engineered the first genetic toggle switch and clock at the turn of the 21st century. Since the development of these first gene circuits, methods for circuit design and construction have improved dramatically, narrowing the gap between concept and proof-of-principle implementation. Engineered genetic constructs have started to leave the lab for the real world where they are being used in applications, including medicine, biosensing, and industrial chemical production. The field of synthetic gene circuits has also grown from focusing on single, isolated circuits to designing complex systems that operate across multiple populations in carefully engineered consortia. In addition, design methods have progressively moved toward including detailed models of the interactions between the host genome and the synthetic gene circuits that it contains in order to better predict circuit dynamics. This article will review some of the most recent advances in gene circuit design and implementation, with a focus on synthetic gene circuits being applied to address real-world problems.