The burden of disease as a result of overweight and obesity calls for in-depth examination of the main causes of behavioural actions responsible for weight gain. Since weight gain is the result of a positive energy balance, these behavioural actions are referred to as 'energy balance-related behaviours' (EBRBs). In the broadest sense, there are only two EBRBs: food intake and physical activity. However, both diet and physical activity are complex behavioural categories that involve a variety of actions. This article discusses the potential problems and opportunities related to the assessment of cognitive determinants of energy intake and energy expenditure behaviours. We argue for the necessity of studying determinants of EBRBs within an energy balance approach, i.e. focusing on energy input as well as output, instead of only studying dietary change or physical activity behaviour. As a result, however, theoretically sound questionnaires assessing determinants of EBRBs are likely to annoy respondents. It is especially the measurement of the behaviours and the use of belief-based constructs that cause questionnaires to be long, which may lead to low response rates and invalid data. In this article, we propose a careful and systematic consideration of the inclusion or exclusion of measures of cognitive determinants. First, if studies show that an EBRB is strongly influenced by environmental factors and is not or only to a minor extent under intentional control, measurement of cognitions is of little use. Second, only when we have proof that attitudes, norms and perceived behavioural control predict intentions, should we aim to assess the underlying beliefs. Third, since assessment of beliefs results in similar or better prediction than using belief-valuation combinations, we should not 'annoy' respondents with valuation items. Finally, we argue that the traditional paper-and-pencil survey is still the most reliable and practical data collection method. However, pilot studies applying computerised adaptive methods to determinants of EBRBs are encouraged.