There is increasing recognition that relational processes are closely linked to, and may even provide the basis for many higher-order cognitive processes (e.g., Gentner, 2003b; Hayes, Barnes-Holmes, & Roche 2001). The primary aim of the current doctoral research was to explore the relationship between relational processes and human intelligence, focusing particularly on relational flexibility. The current research employed a relatively new methodology, called the Implicit Relational Assessment Procedure (IRAP; Barnes-Holmes et al, 2006) The IRAP is a computerised task, which requires participants to respond quickly and accurately in ways that are either consistent or inconsistent with previously learned relations. Response latencies are recorded on both consistent and inconsistent trials. A difference-score is also calculated by subtracting latencies on consistent trials from those on inconsistent trials. The difference-score therefore, provides a relatively “pure” measure of relational flexibility. The current work comprises of four correlational studies. In Study 1 participants completed before/after and similar/different relational tasks, presented on the IRAP. They then completed the Kaufman Brief Intelligence Test (K-BIT; Kaufman & Kaufman, 1990). Study 2 was a replication and extension of Study 1, in which, participants completed the same relational tasks, but were subsequently exposed to extensive and comprehensive cognitive abilities measures, including the WAIS-IIIUK (Wechsler Adult Intelligence Scale – Third UK Edition: Wechsler, 1997). Study 3 involved the presentation of more complex relational tasks, including verbal and perceptual analogies and arithmetic. Study 4 returned once again to more basic relational frames. Specifically, in this study hierarchical and iv comparative relations were targeted, and subsequently the AH4 (Alice Heim 4; Heim, 1970) and the WAIS-IIIUK were administered. In general the results of the studies demonstrated that participants with higher scores on the intelligence test were not only faster at responding relationally, but also demonstrated a greater degree of relational flexibility than those with lower IQ scores. Interesting however, the more complex relational tasks presented in Study 3, produced fewer significant correlations with the intelligence measures than the relatively basic relational tasks. The results also indicated that particular types of relational frames predict performances on certain types of cognitive tasks. Overall, the data obtained from the current research facilitate a greater understanding and greater specification of the processes underlying human intelligence. They also highlight the utility and sensitivity of the IRAP for investigating relational responding. Furthermore, the results suggest that targeting the fluid and flexible development of relational repertoires, may be crucially important in terms of promoting intelligent and creative behaviours in educational settings.