UNLABELLED: Background: Camouflaging (also referred to as "masking") is a commonly reported strategy used by autistic adults in everyday life to help them cope in social situations. Autistic adults report that camouflaging can have a devastating effect on mental health and well-being, yet little is known about the lived experiences of camouflaging and its impact. Methods: We designed an online survey in partnership with autistic adults, to explore the experiences of camouflaging and its impact on mental health. Participants self-reported the lifetime experience of camouflaging, where they camouflaged the frequency and length of time spent camouflaging. Four open questions allowed participants to elaborate their answers to the closed questions on frequency and length of time, and subsequently any positive and negative aspects of their experience of camouflaging. Two hundred seventy-seven autistic adults who self-reported a diagnosis of an autism spectrum condition (128 female, 78 male) or self-identified as autistic (56 female, 15 male) were included in the analysis of qualitative responses to the open-ended questions. Findings: We thematically analyzed participant answers from the open questions. Three main themes emerged. First, "dangers of camouflaging" described how the amount of time spent camouflaging led to exhaustion, isolation, poor mental and physical health, loss of identity and acceptance of self, others' unreal perceptions and expectations, and delayed diagnosis. Second, "positive aspects of camouflaging" included greater access to social spaces, and protection from harm. Camouflaging was, therefore, seen as necessary to survive in a world designed for the neurotypical majority. Third, autistic adults described being diagnosed and accepted for who they are as reasons for "why I don't need to camouflage like I used to." Conclusions: Time spent camouflaging is what seems to be most damaging for the participants' mental health. The main reason reported for needing to spend so much time camouflaging is society's lack of awareness and acceptance of autism. LAY SUMMARY: Why is this study being done?: Many autistic adults report that they need to camouflage their autistic behaviors to help them "fit in" and cope in social situations with non-autistic people. This is because society is not as aware and accepting of autistic people as it needs to be. We also know that for most autistic adults camouflaging is exhausting and damaging for their mental health. This study is important, because researchers have not studied camouflaging enough to know what it is like for autistic adults to camouflage in their everyday lives and to understand the impact that camouflaging has on their mental health.What was the purpose of this study?: We wanted to ask autistic adults about their positive and negative experiences of camouflaging. This is important because it will help professionals better understand why autistic adults camouflage, and better support the mental health needs of autistic adults. This increased understanding may also help society become more aware and accepting of autism. If this happens, autistic adults will not need to camouflage as much. Not having to camouflage as much could also help prevent and reduce mental health problems in autistic adults.What did we do?: We asked autistic adults with a clinical diagnosis and those who self-identify as autistic to complete an online survey. The survey asked questions about mental health, self-injury, suicidal thoughts, and suicidal behaviors. One part of the survey asked questions about camouflaging. If research participants said they camouflaged or masked their autistic characteristics to cope with social situations, they would then be asked about when and why they camouflage, and about the positive and negative consequences of camouflaging.What did we find?: We found that autistic people confirmed that they camouflage because of a lack of awareness and acceptance of autism in society. We also found that both autistic males and females camouflage. Although some autistic adults said that "everyone" camouflages, they thought that autistic people spent much more time than non-autistic people camouflaging in their everyday lives. Spending lots of time camouflaging was what was most damaging for autistic adults' mental health. Although most autistic adults thought that camouflaging was damaging to their mental health, some thought that it helped them too.How will knowing this help autistic adults?: Our results suggest that it is important to reduce pressure to camouflage. This could help prevent high rates of mental health problems in autistic people. Our results suggest that this can be achieved if wider society becomes more aware and accepting of autistic people. Our results also suggest that reducing pressure to camouflage could benefit everyone in society.