This paper provides a summary of applied research undertaken at BAE Systems Advanced Technology Centre to examine the validity and reliability of the subjective ISA (Instantaneous Self Assessment) workload tool. An experimental approach was taken which compared the measure with NASA TLX and the Bedford Rating Scale (BFRS) to establish concurrent validity using an aerospace analogous task MAT-B (Multiple Attribute Task Battery). The results indicated that ISA is a valid tool, with reasonable internal reliability, good retest reliability and no detectable task intrusion. Construct validity was also established as high (.82), which was greater than the other competing workload tools. These combined findings suggest that ISA is a valid subjective workload tool.