How do members of the lay public understand ionizing radiation and its effects? How do they define ionizing radiation, and to which other concepts do they connect it? Do they perceive danger and risks? Do they know how to protect themselves? What do they think of various applications of ionizing radiation? Where and how does the public lodge confidence or doubt? What is the demand for information and transparency? We applied the mental models approach to investigate these questions in four European countries (France, Poland, Romania, and Slovenia; 63 respondents), providing a rich picture of how the lay public perceives ionizing radiation. Mental models are messy and overlapping but meaningful and useful schemas, resulting from a person's multidimensional experience and relation to the surrounding natural and social world. Collectively, members of the lay public (independently of their education or background) possess a non-negligible amount of knowledge on the topic of ionizing radiation risk, and hold strong views on related concepts. Mutual understanding between specialists and potentially affected citizens may be a critical pathway to build a shared and effective risk culture supported by accurate, two-way risk communication and inter-actor relationships.