Cells use genetically-encoded molecular circuits to execute diverse biological functions. We are developing novel tools of synthetic biology that allow us to construct regulatory circuitry inside living cells that recapitulate complex functions like those seen in nature. In this talk, I will describe how we use this approach to achieve three objectives. First, I will demonstrate how synthetic reconstitution provides a powerful way to understand fundamental principles of regulatory networks, which we have applied to guide discoveries in eukaryotic transcription regulation and epigenetics. Second, I will describe our advancements in developing synthetic circuit technologies that enable precise, instructive control of therapeutic human cell function to address challenges in emerging cellular therapies, such as CAR-T cells for cancer. Finally, I will share a future vision of how synthetic reconstitution can be used to engineer cells with little or no intrinsic therapeutic potential into powerful and scalable engines for generating custom, therapeutically-relevant molecules. Overall, by learning how to build biological systems from scratch, our broad goal is to connect the basic molecular building blocks of life to complex cellular behavior and ultimately to clinical applications.