The use of Rasch measurement techniques with data from developmental psychology has provided important insights into human development (e.g., Bond, 1997, 2003; Dawson, 2002 a, b;). In particular, Rasch methods support investigations into what has been, up until now, intractable theoretical and empirical problems. Research into the development of formal operational thinking using the Rasch model (Bond 1995 a, b; Bond and Bunting, 1995; Bond and Fox, 2001) substantiates important aspects of the original theorizing of Piaget (Inhelder and Piaget, 1955/1958), which was based wholly on qualitative structural analyses of children's problem-solving responses. Common-person equating of student performances has been used across different formal operational thinking tasks to estimate the relative difficulties of tasks measuring the same underlying developmental construct (Bond, 1995b; Bond and Fox, 2001). Repeated person performance measures on the same task have been used in order to estimate cognitive development over time. Rasch measurement estimates of cognitive development do not exceed 0.5 logits per annum (Bond, 1996; Endler, 1998; Stanbridge, 2001); a result that has been estimated independently in two large research projects in the United Kingdom (Shayer, 1999) and in Papua-New Guinea (Lake, 1996). Interestingly, difficulty differences (decalage) between tests of formal thought are as large as 2.0 logits (Bond, 1995a; Bond, 1996; Bond and Fox, 2001), confounding attempts to differentiate development from decalage. Given the problems and possibilities raised by the Rasch measurement quantification of cognitive development, this article canvasses the promise of using Rasch modelling techniques to investigate systematically these fundamental aspects of human cognitive performance.