General terms and conditions can be deemed unreasonably disadvantageous and thus ineffective according to Section 307 (1) Sentence 2 of the German Civil Code (BGB), even if they are not clearly and understandably formulated. This requirement for transparency was anchored in the general clause of the law on terms and conditions during the modernization of the law of obligations in 2001, after having previously been developed as a principle of case law. The author initially examines the methodological and doctrinal implications of this codification on the basis, content, and limits of the transparency requirement. The focus is on the relationship between substantive content control and formal transparency control. Another emphasis is placed on the principles for determining opacity and its unreasonableness. Furthermore, the positioning of transparency control within the overall system of terms and conditions control is defined, particularly in relation to interpretation (rule of ambiguity) and delineated in the area free from content control, specifically for declaratory and performance-defining clauses. Finally, the developed principles are exemplified through several practical questions in the specific area of employment contract control.