This paper offers a comprehensive analysis of the information from the Survey of Adult Skills (PIAAC) regarding adults with low literacy and numeracy proficiency. The paper first describes the demographic and socio-economic characteristics of these populations. Although, they are more likely than the rest of the population to exhibit certain characteristics, such as lower levels of educational attainment, lower rates of unemployment or more disadvantaged backgrounds, adults with low literacy are found in among all socio-demographic groups and in all walks of life. The paper then explores the frequency with which adults with low proficiency engage in the reading, writing and numeracy practices and the relationship between these practices and a range of social and economic outcomes. For most outcomes, levels of engagement in literacy practices appear to be as strong predictors as proficiency, indicating the importance of encouraging more intense use of these skills both in and outside of work. The unique data from the Survey of Adult Skills regarding performance on the simple reading tasks (the so called "reading components") is also analysed. Adults with low proficiency are found to be able to easily recognise commonly used words in printed form but often have difficulty with processing the logic of sentences and reading extended passages for basic meaning. Adults with low proficiency are considerably less likely than their more proficient peers to participate in formal or non-formal adult education or training programmes, which is mostly due to the socio-demographic and employment characteristics of this population. However, the lower participation rates among the low proficient adults does not appear to be a consequence of their lack of motivation as much as of the presence of various obstacles to participation, such as lack of time and the cost of training. PIAAC Methodology for Measuring AET [Adult education and training] and two supplemental tables are provided in an annex. Individual chapters contain references.