Sexually transmitted infection (STI) stigma persists and can delay or prevent seeking care at sexual health clinics but help-seeking in response to genito-urinary symptoms is not well-understood and often clinically framed. I explore perceptions and social representations of STIs and how these influence lived experiences of genito-urinary symptoms and help-seeking responses. I focus on non-attendance at specialist sexual healthcare services in women and men in Britain. This is an explanatory sequential mixed methods study. I conducted secondary analysis using data from Britain’s third National Survey of Sexual Attitudes and Lifestyles (Natsal-3), a probability sample survey carried out 2010-2012. Prevalence estimates and logistic regression models were used to calculate population patterns of genito-urinary symptoms and help-seeking preferences and behaviour. Sequential semi-structured interviews took place 2014-2015 with 16 women and 11 men who had participated in Natsal-3 and reported symptoms in the past month and never having attended a sexual health clinic. I developed and implemented an image-elicitation method to produce data about social representations of STIs and used thematic mapping and framework analysis to understand perceptions of STIs, individual sense-making processes and lived experiences of symptoms and helpseeking responses. I integrated key findings using matrices and joint displays to connect and extend understanding of the phenomena. From the semi-structured interview data, dirt emerged as a common social representation of STIs and key component of STI stigma. My findings suggest there were moral and physical dimensions to dirt, which were often conflated by participants, and a range of strategies to deal with STI dirt were described: silencing and concealing; distinguishing STIs from other health issues; preventive and helpseeking strategies. Dirt framed participants’ experiences of symptoms and helpseeking which were themselves often concealed, silenced and dissociated from STIs. Survey analyses showed symptoms were more commonly reported by women (22%) than men (6%) and I proposed a new model - the Cause-Concern Cycle - to explain how participants interpreted sensations and symptoms based on qualitative data. Symptoms are sensations that cause concern and have a suspected underlying medical cause. The meanings attributed to experiences mediated subsequent helpseeking responses. Non-attendance at sexual health clinics in the past year was common in both women (86%) and men (88%) as reported in Natsal-3 but participants sought help from healthcare and other sources to gain control and emotional reassurance rather than prioritising medical needs, determined from analysis of semi-structured interview data. The data highlight that current sexual health service provision is sufficient in terms of accessibility and choice and convey a number of messages for sexual health policy makers about managing untreated STIs and unmet sexual health needs. However, these are discussed in the context of the current climate of huge funding cuts to public health budgets which is already drastically altering the landscape of sexual health in Britain.