With the rise of microservices, the execution environment of many cloud applications has become a set of virtual machines or containers connected by a flexible and feature-rich virtual network. We argue that the implementation of such virtual networks should be completely application-specific and not layered on top of general-purpose network abstractions from the Internet age. Such layering tends to more than double the latency and CPU usage of applications. We propose application-defined networks in which developers specify network functionality in a high-level language and a controller generates a custom distributed implementation that runs across available hardware and software resources. Experiments with a preliminary prototype suggest that, compared to the state of the art, ADN reduces latency by up to 20x and increases the throughput by up to 6x.