The river deltas of the arctic coast of Siberia fall into three main categories: deltas infilling gulfs or inlets, e.g., the Taz and Pur, deltas infilling rivermouth lagoons, e.g., the Yana and Indigirka, and deltas which are advancing out from an open sea coast, e.g., the Lena. Each type of delta is discussed in terms of its geometry and channel network, distribution of discharge, lengthening at the mouth, geomorphology of the delta, the hydrodynamics of the area to seaward of the delta, and the boundaries of the rivermouth area and of the delta. A distinction is also made between the “old delta”; and the “present delta”; on the basis of channel geometry, fluvial and deltaic processes and the geomorphology of the two areas. Present processes of delta formation on the arctic coast of Siberia are proceeding at a relatively slow rate, a phenomenon which is ascribed mainly to the retarding effect of permafrost.