This study advances the thesis that prayer can be understood as the key to the fundamental structures of Karl Barth's Church Dogmatics. A particular focus is on the question of how humans are to be perceived as active and receptive subjects in the encounter with God. In the development of the argument, therefore, a key function is attributed to the concept of reciprocity. In this horizon, the relationship is analysed between the petition - which for Barth is central - and the response (as a realisation of the divine relationship). In this, the encounter with God is realised positively through the petition expressing human neediness by the prayer turning into praise. Here, a fundamental hermeneutic structure of Barth's is revealed - Barth wants to do justice to the religious experience in prayer by the very act of not thematising it. It is, however, rehearsed indirectly through the dogmatic statements of his theology.