In our contribution, we investigate the relationship between social investment (SI) and lifelong learning (LLL), discussing how LLL can be integrated theoretically within a SI approach, by focussing on the ideational principles and policy strategies. First, we describe the SI and LLL perspectives, reviewing their foundations and main principles. These approaches present overlaps, but also differences related to the scope and aims of interventions. Moreover, both the critical debates on SI and LLL stress the relevance of ambiguities that can be traced back to the existence of a narrower functionalistic understanding and market-lead human capital approach. This view is contrasted with a holistic comprehension of inclusion taking into account issues of social participation and human capabilities. Second, we discuss institutional and contextual complementarities as preconditions for the effectiveness of SI policies, to be identified in the complex and time-framed interface among the labour market, the education system and the welfare state. By doing this, we argue that LLL policies play a key role in a social investment strategy, specifically addressing the time dimension by means of coherent interventions over the life course. We suggest that further research may investigate, according to this conceptual framework, the specific combinations of SI and LLL policies within different national contexts.