Chapter 2 presents the research on parameters within GB Theory. After classifying the parameters from Rizzi’s (2014) list into five different types, the chapter reviews the null subject parameter (Rizzi 1978), the parameterization of Subjacency (Rizzi 1978), and the parameter of S′-deletion (Chomsky 1981a). Some influential proposals which were originally formulated as non-parametric systematic differences are then reviewed, namely Huang’s (1982) insights on the locus of wh-movement, Kayne’s (1983) treatment of preposition stranding and epistemic verbs, and den Besten’s (1983) account of V2. The next section discusses Hale’s (1983) parameterization of the Projection Principle, Baker’s (1988) analysis of incorporation processes, Pollock’s (1989) split INFL hypothesis, and lastly, Koopman and Sportiche’s (1991) proposed cross-linguistic difference in nominative Case assignment. The chapter ends by covering some influential proposals concerning language acquisition, that is, Hyams’s (1986) hypothesis that parameters have preset default values, Manzini and Wexler’s (1987) parameterized notion of governing category, and finally, the latter’s link to the Subset Principle.