The KMOS Lens-Amplified Spectroscopic Survey (KLASS) is an ESO Large Programme that uses the KMOS infrared spectrograph to investigate the role of low-mass galaxies at several epochs of cosmic time. KLASS has targeted galaxies behind massive clusters, using gravitational amplification and stretching to observe galaxies that are intrinsically very faint. By pushing KMOS to the limits of its capabilities, we have obtained new constraints on the timescale of the reionisation process, finding that the intergalactic medium was almost completely neutral at a redshift of around 8, and that turbulence plays a major role in shaping low mass galaxies at intermediate redshifts (0.5 < z < 2).