This study examines beliefs about helmet use in adult cyclists from 17 countries participating in the COST Action TU1101 Project (Helmet Optimization in Europe - HOPE). A total of 5797 respondents were included in the analysis after applying eligibility criteria and data cleaning. Cyclists' beliefs were assessed by 25 items using a 7-point Likert scale. These items were factor analyzed resulting in a four-factor solution (Factor 1: Perceived Benefits and Risk Reduction, Factor 2: Perceived Disadvantages and Barriers to Helmet Use, Factor 3: Perception of Group Norms, and Factor 4: Situation-Dependence of Helmet Use). Results show that both beliefs and helmet wearing behavior differ according to some demographic and cycling-related factors, such as gender, frequency of bicycle use and type of bicycle used. Factor 3 (Perception of Group Norms) and Factor 2 (Perceived Disadvantages and Barriers to Helmet Use) were among the strongest predictors of helmet use, even after controlling for demographic and cycling-related variables. The findings suggest that helmet use promotion should focus primarily on normative beliefs and on the reduction of perceived barriers. Beliefs about helmet use can vary in accordance with the cyclist's profile, as well as cultural and contextual factors. Assessment of beliefs in specific settings and populations will provide a better basis for planning interventions. The limitations of the present study are mainly related to the instrument used (online survey) and to the differences in sample sizes across countries. (C) 2019 Elsevier Ltd. All rights reserved.