People aged 55 and older are one of the fastest growing demographic groups in much of the developed world. While they have seen tremendous changes in technology and everyday living over their lifetime, some of the biggest societal changes are those that they themselves are helping to make. In the U.S., these mature individuals can expect to live several decades beyond the traditional retirement age of 65-67, and they will likely be able to do so in much better health compared to earlier generations. While a percentage of these individuals will choose full-time retirement in their mid-sixties, a large and growing percentage will want or need to have paid work. Within this group are very diverse interests and needs. Some mature workers will want to stay in their current jobs as long as possible, while others may want to migrate to a different kind of job as a way to transition to full-time retirement (for example, they may wish to find a job with shorter hours, less stress, or fewer physical demands). Some mature workers may want to find something to do in this next stage of their work lives that is completely different than anything they have done in the past. Some in pursuit of such "encore careers" may want anything new, while others may seek to do something they have always dreamed of or something that gives back to the community. Still other mature workers may find that their lifelong jobs have disappeared. Facing unexpected unemployment, they will need to find a replacement job that may be in another industry. In 2009, the U.S. Department of Labor supported a three-year Aging Worker Initiative (AWI). This initiative acknowledged the importance of mature workers to the economy and the need to provide them with a different kind of help in preparing for new jobs and careers. The AWI funded 10 sites to test new models of serving mature workers. Over the course of this three-year initiative, the Council for Adult and Experiential Learning (CAEL) has provided technical assistance to these sites under a grant from The Atlantic Philanthropies. The Council on Competitiveness assisted as well during the first two years of the initiative. The experiences of the 10 sites have underscored for us the importance of supporting the labor force participation of mature individuals in a deliberate way. As this initiative comes to a close, CAEL invited leading experts on the mature workforce to help us showcase the work of the AWI grantees alongside their own research findings. Collectively, the papers commissioned for this publication address the factors that are causing more mature individuals to continue working, the range of occupations currently held by mature workers, the value of the mature workforce to our economy, the factors that most influence employer retention and hiring as they face changing workforce demographics, the difficulties workers face as they make often irreversible decisions about when to retire, and public sector strategies that support mature workers.