Obligations to cooperate in tax proceedings are of central importance for taxpayers. In addition to outlining the content, scope, and limits of these obligations, this work also examines their relationship with criminal, constitutional, and European law, as well as the legal consequences of a breach of cooperation obligations. The focus is on documentation requirements in cross-border transfer pricing cases, which have gained attention in recent years due to their practical relevance in preventing income shifting to low-tax countries, drawing the interest of jurisprudence, tax authorities, and literature. An analysis of the legal situation prior to 2003, particularly the ruling of the Federal Fiscal Court from October 17, 2001, is included, along with an examination of the subsequent legislative developments considering administrative principles, procedures, and interpretations in literature.