Background: Front-of-pack (FOP) nutrition labelling has been proposed as a tool for helping consumers make healthy choices. Before determining its effects on consumer behaviour, factors involved in its use must be elucidated, i.e. understanding and acceptability on the part of the consumer. Among five FOP labels, we sought to determine which formats were most easily understood and accepted by a large sample of adults. Methods: Among 39 370 adults who participated in the French Nutrinet-Sante cohort study, understanding and indicators of acceptability (attitude, liking, visual attractiveness and perceived cognitive workload) were measured for five FOP labels: The currently used 'multiple traffic lights' (MTL) and 'simple traffic lights' (STL), and the 'colour range' logo (CR), the 'green tick' and the PNNS logo. We investigated the contribution of the different elements to consumer perception of FOP labels using multiple correspondence analyses. Results: Over half of the sample population showed a high level of understanding and perceived no discomfort in terms of the different logos. Label formats were positioned along an acceptability gradient ranging from acceptance to rejection, consisting of 'liking', 'attractiveness' and indicators of perceived cognitive workload. MTL was significantly more often liked and was viewed as reliable and informative. MTL, STL and the green tick performed better than the CR and PNNS logos in terms of ease of identification and comprehension. CR was clearly the least appreciated and it had the most complex format. Conclusions: Consumers prefer FOP labels which give complete, reliable and simplified information on the nutrient quality of foods.