The theory of planned behaviour is a well-known theory within social psychology. According to this theory people’s attitude towards the behaviour, their subjective norm and their perceived behavioural control determine their behaviour (a defined action) indirectly via their intention (a willingness to try to perform the behaviour). Attitude towards the behaviour is determined by behavioural beliefs, which are beliefs about the likely consequences of the behaviour (behavioural belief strength), weighted by the evaluation of how good or bad these outcomes would be (outcome evaluation). Subjective norm is determined by normative beliefs, which are beliefs about what important others think of the behaviour (normative belief strength), weighted by the motivation to comply with these important others (motivation to comply). Perceived behavioural control is determined by control beliefs, which are beliefs about factors that may facilitate or impede performance of the behaviour (control belief strength), weighted by the perceived power of these factors (control belief power). A positive attitude and subjective norm together with a large perceived behavioural control results in a strong intention to perform the behaviour. Given enough actual control over the behaviour, people are expected to carry out their intention as soon as an opportunity is given. For behaviours over which people have incomplete volitional control it is also useful to consider perceived behavioural control as a codeterminant (together with intention) of the behaviour. The relationship between perceived behavioural control and behaviour is however dependent on the accuracy of people’s perception of their control over the behaviour. Within traffic psychology the theory of planned behaviour has been successfully used as a frame of reference to predict and explain behaviours such as drinking and driving, dangerous overtaking, close following, lane discipline and speeding. A short review of different studies using the theory of planned behaviour will be presented and pros and cons with the theory will be discussed.