In Finland, the proportion of 45- to 64-year-old people in the work force is expected to increase from 31.1 to 40.1% between 1980 and 2000. A multidisciplinary study was carried out to test the factors of work, health, work ability, functional capacity, and perceived strain as criteria for determining new concepts for retirement age. The first phase of the project comprised cross-sectional studies in which the age of the 6257 working subjects ranged from 45 to 58 years. The second phase covered a four-year follow-up of these same subjects. The main results of the 15 various studies are given in separate articles. Recommendations for improving the work and promoting the work ability of the aging worker have been made on the basis of the findings of the project.