The goal of this research synthesis is to evaluate the validity of the claim that there are fundamental limitations on the ability of adolescents to engage in critical thinking. Critical thinking is defined as an analytic, focused cognitive activity aimed at understanding phenomena at a root rather than a superficial level; a type of thinking closely though not exclusively associated with formal thinking as described by Piaget. This paper examines what fundamental cognitive developmental limitations have been claimed or implied regarding adolescents' abilities to engage in critical thinking, what empirical evidence has been used to support or refute these claims, what empirical evidence exists to demonstrate that adolescents are able to engage in such thinking, and what recurring themes emerge from consideration of all this evidence. Evidence is reviewed within four broad headings: (1) Piaget's formal operations; (2) biopsychological constraints; (3) individual differences; and (4) cognitive processing analyses. It is concluded that there is no persuasive evidence of fundamental constraints on the ability of early adolescents to engage in critical thinking. Sixty-three references are included. (NB)