Low literacy remains a problem across the developed world. Health literacy has emerged as a specialist literacy, although its conceptualisation and measurement overlap with those of functional literacy. The social practice view, which conceptualises literacy as an activity rather than a skill, is useful in examining literacy-related practices and demands in healthcare contexts. Associations between low functional or health literacy and poorer health outcomes have been established in the research literature but studies have included participants whose low literacy may be obvious to healthcare providers and others. This thesis presents first a systematic review that focuses on literacy and health outcomes in the ‘hidden population’ with low literacy, defined as those of working age whose first language is that of their resident country but who score low on literacy measures. The review concludes that there is a link between low literacy and poorer health in this ‘hidden population’ but that potential pathways have not been explored to any extent. The thesis then presents a primary research study which sought to explore links between low literacy and poor health from the perspectives of people with low literacy; and how the health service might respond to address the issues raised. The findings from 25 in-depth interviews and 2 focus groups with adult learners highlighted the various ways that people with low literacy struggle with written and spoken communication in clinical encounters and in self-care activities. They further revealed that stigma frequently plays a central role in the social practices of those with low literacy, affecting their mental wellbeing and social relationships, including those with healthcare staff. The potential solutions identified for service providers would benefit from piloting and evaluation to help create a literacy-sensitive health service, which could improve engagement; enable self-care and enhance capabilities for health in those with low literacy.