Abstract: A thermal transfer medium comprises a substrate bearing on at least part of one surface thereof a coating of a thermally transferable ink comprising binder and a plurality of discrete particles of optically variable pigment (OVP) dispersed in the binder. The ink can be printed using convention thermal printing apparatus and methods to produce on an item such as a personalised card an image the optical properties e.g. colour of which vary depending on the angle of viewing, thus constituting a security feature that is difficult to replicate or simulate, enabling the authenticity of the image-bearing item to be readily checked by visual inspection. The invention also concerns a method of malting the thermal transfer medium, a method of thermal transfer printing using the medium and the resulting printed material.