Synthetic biology, the science of engineering complex biological systems with novel functions, is increasingly fascinating researchers across disciplines who gather to design functional biological assemblies in a rational and systematic manner. Although initial success stories were based on reprogramming prokaryotic and lower eukaryotic cells, the design of synthetic mammalian gene circuits is becoming increasingly popular because it promises to foster novel therapeutic opportunities in the not-so-distant future. Here, we discuss the latest generation of mammalian synthetic biology devices assembled to form complex synthetic gene networks, such as regulatory cascades, logic evaluators, hysteretic circuits, epigenetic toggle switches, time-keeping components, drug discovery tools, and "cell phone" units. We further highlight how such circuits could be interconnected to achieve higher-order control networks such as synthetic hormone-like communication systems in animals or synthetic ecosystems with dynamic interspecies crosstalk.