The ML-CIRRUS mission deployed the novel high-altitude long-range research aircraft HALO to get new insights into the nucleation, life cycle and climate impact of natural cirrus and anthropogenic contrail cirrus. The ML-CIRRUS mission with the G5 research aircraft HALO combined an in situ/remote sensing payload including a suite of direct state-of-the-art cloud instruments and a novel aerosol and ice residual, trace gas and radiation instrumentation as well as a high-spectral-resolution water vapor lidar. The aircraft observations were assisted by remote sensing observations from satellite and ground and by numerical simulations to predict cirrus and contrail cirrus occurrence. ML-CIRRUS will provide a comprehensive data set on natural cirrus and aircraft-induced cloudiness for cloud process studies and climatological considerations.