As a consequence of the rapid growth dynamics at low temperatures, a rough 4He solid-liquid interface can support wave-like motion that is analogous to the capillary or gravity waves found at liquid surfaces. These deformations or ''crystallization waves'' move along the interface by alternately growing and melting the solid in the direction of motion. These ''crystallization waves'' can be used as a direct and sensitive probe of the interfacial stiffness and the interfacial growth dynamics.
I will first introduce a few simple concepts that are important for understanding the shape and growth of crystal surfaces (surface free energy and the roughening transition). These principles will be illustrated with photographs of lead, gold and helium crystals. I will then show how the unique properties of liquid and solid helium make it possible for ''crystallization waves'' to be excited at the solid-liquid interface. I will conclude with a description of our measurements of the interfacial growth coefficient for crystals grown from pure 4He and 3He-4He mixtures.