We have developed optical, interferometric methods for investigations of interfaces at ultra low temperatures. In our scheme conventional optical windows are avoided: laser illumination (He-Ne) is guided into the cryostat via a single-mode optical fiber and images are taken using a CCD sensor mounted inside the 4-K vacuum can. A real-time video camera has been successfully used in investigations of superfluid3He down to 0.6 mK whereas a slow-scan camera has been employed for optimal contrast in low-intensity imaging of liquid/solid interfaces (reflection coefficient ∼10-6). The investigated topics include (1) superfluid3He surface in rotation and during rapid deceleration, (2) hydrodynamics of thin superfluid3He layers, (3) superfluid/solid interface in4He, and (4) wetting of superfluid4He by normal3He in phase separated mixtures. A vertical resolution of 10 nm and even below has been achieved in these studies.