The primary goal of optical microscopy is to visualise and thereby understand microscopic structure and dynamics. Dramatic developments over the past decades have enabled routine studies down to the single molecule level and structural observations far beyond the limits defined by the diffraction limit through the use of fluorescence as a contrast mechanism. Despite its many advantages, one of the fundamental limitations of fluorescence detection is the frequency with which photons can be emitted and thus detected. As a consequence, although images and even movies of single molecules have become commonplace, imaging speed remains limited to few to tens of frames per second by the quantum nature of single emitters. The result is a considerable gap between the rate at which dynamics can be recorded and the underlying speed of motion on the nanoscale.Here, we introduce an alternative approach to optical microscopy that relies on the ultra-efficient detection of light scattering, rather than fluorescence, called interferometric scattering microscopy (iSCAT). We show that iSCAT is capable of following the motion of nanoscopic labels comparable in size to semiconductor quantum dots with nm accuracy down to the microsecond regime, the relevant timescale for a majority of nanoscopic dynamics. Thereby, we are able to address a surprising variety of fundamental questions in molecular biophysics ranging from the mechanical properties of DNA, the mechanism of molecular motor processivity and anomalous diffusion in bilayer membranes and its possible origins. We also demonstrate the potential of iSCAT for label-free, all-optical biosensing and imaging at the single protein level.