The mental effort required to perform a simultaneous interpreting task or the cognitive load generated by it has attracted the interest of many a researcher in the field. To date, however, there is little agreement on the most suitable method to measure this phenomenon. In this contribution, the author sets out to discuss four of the most common methods of measuring cognitive load and the way in which they have been applied in interpreting research, providing examples for each and highlighting their respective advantages and disadvantages. The main focus of the contribution will be on pupillometry, a psycho-physiological method the author deems to be among the most promising approaches to objectively measure cognitive load during simultaneous interpreting in real time.