In this paper, we describe recent results in mid-infrared heterodyne detection using quantum-cascade (QC) lasers as local oscillator (LO). In the 9 microm range, the heterodyne detection technique was first developed with CO(2) lasers and then with Pb-salt diode lasers. Quantum-cascade lasers are promising high quality tunable mid-infrared sources. We developed a quantum-cascade laser based heterodyne spectrometer. Atmospheric absorption spectra of ozone are presented.