Purpose of the Review: The increasing penetration of distributed energy sources into the electricity system requires greater customer engagement in providing new flexibility services. This article reviews the main behavioural barriers and existing gaps related to customer engagement in emerging flexibility markets. Recent Findings: Greater attention should be put on collecting more experimental evidence from the field to finalise the development of appropriate engagement strategies that can avoid unexpected customer reactions, especially when different incentives and information are provided. Summary: Based on an extensive review of the literature, we identified three overarching groups of behavioural barriers to customer engagement in flexibility markets, namely: (1) lack of awareness, (2) lack of skills to process information, and (3) inertia. Furthermore, there is still no consensus on the effectiveness of incentivisation mechanisms and information strategies that can enhance customer engagement.