The coercivity of a magnetic material is a crucial parameter in determining its use in recording. The apparent value of this quantity is found to depend upon the time scale of interest. A method of measurement that observes changes of magnetization on a short time scale will yield a larger coercivity than one that measures changes over a longer time scale. The cause of this phenomenon is thermally assisted reversal of magnetic domains. Thus, for a magnetic recording material, the coercivity relevant to a high-speed writing process is greater than that relevant to transition broadening and long-term storage stability. The difference between the short-and long-term coercivities becomes more pronounced as the size of the magnetic switching unit becomes smaller. Therefore, these kinetic effects will become increasingly important in determining the ultimate limitations of future high-density media. Data are presented that illustrate the variation of coercivity with time scale in presently used and experimental recording materials.