Adaptive Cruise Control (ACC) is a system that, through the automation of the longitudinal driving task, aims to increase drivers’ comfort. Previous studies into ACC showed that behavioural adaptations might occur following its use. Those studies were mainly conducted with drivers who had never used the system before and, to date, little information is available on the behaviour of actual ACC users. Hence, this chapter describes the main findings obtained from focus group discussions and a small scale naturalistic Field Operational Test (nFOT) performed with early adopters. The findings illustrate that behavioural adaptations to the system emerged during both studies and that an improper driver’s mental model of the system might be among the triggering causes.