We study theoretically the formation of dilute phases of spherical (micelles), cylindrical (vermicelles), and planar (lamellae) aggregates of copolymer chains in a selective solvent. In the case of diblock copolymer chains, micelles are formed for all values of the block asymmetry in the regime where dilute phases are predicted to occur (N(A) much less than N(B)3/2, with N(A) and N(B) the polymerization indices of the A-collapsed and B-swollen blocks). For B-A-B triblock copolymers we find that spherical or cylindrical hollow structures might exist in the solvent in a metastable state with a long associated lifetime.