This chapter analyses the scope for policies to raise aggregate employment rates by fostering greater labour market participation among population groups that tend to be under-represented in employment. Under-represented groups are diverse and their relative numerical importance differs from country to country. Nonetheless, women, older workers and less educated workers represent the largest reservoirs of underutilised labour potential in most OECD countries. To what extent are certain population groups, such as women, older workers and less educated workers, trapped in situations of labour market inactivity? Once in employment, do they have career prospects or is there a risk that they will become trapped in low-quality jobs? How would a better mobilisation of these groups help respond to the challenges of population ageing?