This paper describes the system concept and initial results for end-to-end energy-autonomous sensor systems using unmanned aerial vehicles (drones) as agents for power delivery to and data gathering from sensing devices. Such systems may be particularly useful for delay tolerant monitoring scenarios, where sensing devices may be deployed in remote, harsh conditions, often with sparse connectivity, long life and high reliability requirements. This paper discusses the latest advances in wireless power delivery that makes this it possible to fly wireless power delivery systems on drones that have little payload capability.