Quantum states are both powerful and fragile. New frontiers for quantum technologies — for quantum computing, communication, and sensing — are emerging based on quantum superposition and entanglement, but to harness these opportunities, we will need the capability to transfer quantum states between light-based and matter-based encodings. I will discuss the special role that optical cavities can play as interfaces between quantum light and quantum matter. We will then consider two applications: First, I will describe a recent experiment in which we have used cavities to entangle two ions in different buildings on the University of Innsbruck campus. This elementary network provides a testbed for a future quantum internet. Second, we will see that cavities can be used to prepare macroscopic quantum states of the motion of massive objects, offering fundamental insights into the boundary between the classical and quantum worlds. Recently, we have cooled the motion of levitated nanoparticles in preparation for such experiments.