It is widely acknowledged by second language researchers that whether pedagogical rules should or can be explicitly taught is largely dependent on their complexity. Nevertheless, there is no consensus on what rule complexity exactly means. This paper first examines existing accounts of rule complexity and presents a conceptual analysis of the term ‘rule’. It then proposes that ‘complex’ should not be equated with ‘difficult’, but used in a purely structural sense. Specifically a conditional formulation is proposed in which the number of concepts in the antecedent and the consequent, the number of subconditions, and the number of ‘if-then’ connections (subrules) within a given rule domain govern complexity. Finally, a classification of strategies of complexity reduction in foreign language pedagogy is sketched. Throughout, examples will be given from the field of German as a foreign language.