The structure theory of abelian extensions of commutativerings is a subjectwhere commutative algebra and algebraicnumber theory overlap. This exposition is aimed at readerswith some background in either of these two fields. Emphasisis given to the notion of a normal basis, which allows oneto view in a well-known conjecture in number theory(Leopoldt's conjecture) from a new angle. Methods toconstruct certain extensions quite explicitly are alsodescribed at length.