A π 0 1 class is an effectively closed set of reals. We study properties of these classes determined by cardinality, measure and category as well as by the complexity of the members of a class P. Given an effective enumeration {P e : e < ω} of the Π 0 1 classes, the index set I for a certain property (such as having positive measure) is the set of indices e such that P e has the property. For example, the index set of binary Π 0 1 classes of positive measure is Σ 0 2 complete. Various notions of boundedness (including a new notion of a lmost bounded classes) are discussed and classified. For example, the index set of the recursively bounded classes is Σ 0 3 complete and the index set of the recursively bounded classes which have infinitely many recursive members is Π 0 4 complete. Consideration of the Cantor-Bendixson derivative leads to index sets in the transfinite levels of the hyperarithmetic hierarchy.