There has been a recent interest in listening effort as a factor to be taken into account in the audiological clinic. However, the term “listening effort” is poorly determined and needs to be defined before it can be used as a clinical or research tool. One way of understanding listening effort is in terms of the cognitive resources expended during listening. Cognitive capacity is finite and thus if cognitive capacity is used up during the act of listening to speech there will be fewer cognitive resources left to process the content of the message conveyed. We have introduced the term Cognitive Spare Capacity (CSC) to refer to residual cognitive capacity once successful listening has taken place. This extended abstract described the work we have carried out to date on measures of CSC for research and clinical use. In the course of this work we have developed tests to assess the role of memory load, executive function and audiovisual integration in CSC under challenging conditions. When these tests are fully developed, our aim is that they should allow objective individual assessment of listening effort in cognitive terms. Results to date indicate that under challenging conditions, CSC is an arena for executive processing of temporarily stored information; it is related to individual working memory capacity and can be enhanced by hearing aid signal processing.