If the fundamental consideration of domain size were the dominant factor, it would theoretically be possible to store information on magnetic disks with a packing density of at least 1010bits per square inch.To be of practical use, the fundamental design limits of the data channel and its components, the practical magnetic design of the media and recording heads, the mechanical tolerances and wear properties of the entire combination of medium, head and transports, have to be considered. Under practical conditions, current files operate at a density of 2 × 105bit per square inch.This paper reviews the overall interaction of these limiting parameters, and discusses some of them in detail.