The primary research question addressed in this study is: has Warren Buffett's 'meaning-making-in-action' changed over his life, as predicted by developmental theory? Meaning-making is defined within Constructive-developmental theory (Cook- Greuter, 1999,2005; Kegan, 1980,1994; Loevinger, 1976; McCauley et at., 2006; Torbert & Ass., 2004; Wilber, 2000) to mean the internal organising system individuals use to make sense of their experience. The question highlights the importance of vertical development which for Jean Piaget (1954) "was not the gradual accumulation of new knowledge or experience but a process of moving through qualitatively distinct stages of growth, a process that transforms knowledge itself" (McCauley et al., 2006, p. 635). Applying a constructive-developmental framework this study tracks the transformation in Buffett's meaning-making-inaction across thirty-two representative examples from his life that are then compared to one or more of seven developmental action-logics in developmental theory (Cook-Greuter, 2005; Fisher, Rooke & Torbert, 2003; Torbert & Ass., 2004). The study concludes that Buffett's meaning-making-in-action has transformed over his life and in a sequence predicted by the action-logics in developmental theory. This is reflected in a Spearman correlation coefficient of p. 93 which indicates a strong relationship between the predicted order of development and the actual order. While the principle contribution from this study is a methodological one within the field of developmental theory, the research also makes an original contribution to our understanding of the importance of Buffett's development to his effectiveness as 2 a leader. By implication developmental theory has important things to say about how others may develop their leadership as well. Next steps in this research include simplifying the application of the method.