Abstract: Spectroscopy, by providing a direct measurement of the energy spectrum, is a powerful tool to probe matter. Conventional spectroscopy techniques suffer several drawbacks when applied to mesoscopic systems, or artificial quantum coherent atoms. I present an on-chip, Josephson-junction based spectrometer which surpasses state-of-the-art instruments, works between 2-200 GHz, and is ideally suited for probing elementary excitations in mesoscopic systems. I describe the operating principle and design of the spectrometer, show spectra of several superconducting quantum circuits, and outline experiments to investigate single quasiparticles in superconductors.