In a fast-changing and highly inequitable world, lifelong learning is becoming increasingly important, not only as a key organising principle for all forms of education and learning but also as an absolute necessity for everyone. It is particularly important for disadvantaged individuals and groups who have been excluded from or failed to acquire basic competencies through formal schooling. Within a lifelong learning framework, literacy and numeracy are viewed as foundation skills which are the core of basic education and indispensable to full participation in society. This article discusses recent developments in conceptualising literacy as a foundation of lifelong learning. Starting from the evolving notions of adult literacy, the author identifies some current trends, the most important being that literacy is now perceived as a learning continuum of different proficiency levels. Dichotomous states of being either "literate" or "illiterate" no longer apply. She analyses (1) findings extracted from UNESCO Member States' national reports submitted to the UNESCO Institute for Lifelong Learning (UIL) for the "2nd Global Report on Adult Learning and Education"; (2) a desk study of national literacy campaigns and programmes as well as (3) some recent developments in formal education. Her suggested three-dimensional analytical framework considers literacy as a lifelong and life-wide learning process and as part of lifelong learning systems. She draws a number of conclusions for policy and practice of literacy as a foundation of lifelong learning. These conclusions are a timely contribution to the ongoing post-2015 education debate, in particular to the challenge of how to mainstream youth and adult literacy into the implementation of the sustainable development agenda for 2015-2030.