Experimental and theoretical studies of time dependence of the magnetisation are described. The time dependence phenomenon has its origins in thermally activated transitions over energy barriers and is strongly affected by interaction effects. Studies of particulate recording media and thin films are described which give interesting insight into the behaviour of the systems. Consideration is also given to the practical problem of the frequency dependence of the coercivity in particulate recording media which has implications from the point of view of the evaluation of the materials.