PURPOSE OF THE STUDY: This article investigates lay perspectives of the concept of successful aging in young, middle-aged, and older adults from 2 cultures, the United States and Germany, to potentially guide the development of scientific theories of successful aging. The empirical findings are embedded in a comprehensive overview of theories of successful aging and life-span development and offer implications for theory development. DESIGN AND METHODS: Two samples of young, middle-aged, and older adults from the United States (N = 151) and Germany (N = 155) were asked about definitions and determinants of successful aging. Codes were developed to capture common themes among the answers, resulting in 16 categories. RESULTS: Themes mentioned included resources (health, social), behaviors (activities), and psychological factors (strategies, attitudes/beliefs, well-being, meaning). There were striking similarities across countries, age, and gender. Health and Social Resources were mentioned most frequently, followed by Activities/Interests, Virtues/Attitudes/Beliefs, Well-being, and Life management/Coping. Age differences were limited to Growth/Maturation and Respect/Success, and gender differences were limited to Social Resources and Well-being. Educational and cultural effects were limited to psychological factors and Education/Knowledge, which were more often mentioned by U.S. participants and individuals with more education. IMPLICATIONS: Young, middle-aged, and older lay persons from the United States and Germany have quite similar concepts of successful aging, which they view in far more multidimensional terms than do established scientific theories (Rowe & Kahn, 1998). Given evidence that factors mentioned by laypeople do promote successful aging, considering them in more comprehensive theoretical models may enhance our understanding.