This invention concerns measuring cognitive load. A word based input is produced by a person while performing a task (100). Predetermined grammatical features of the word based input are identified (104). Then, the identified grammatical features are weighted and combined to provide a measure indicating the person's cognitive load (106). The use of grammatical features represents a divergence from the known methods of providing a measure of a person's cognitive load. It is an advantage of the invention that by concentrating on the grammatical features it is able to provide an objective, non-intrusive measure of the person's cognitive load. Aspects the invention include a method, a computer system and software that are used to perform the method.