This article addresses the accountability deficit of executive agencies. It argues that the acclaimed deficit consists of two elements: the inability of democratic principals to hold autonomous agencies accountable and the insufficiency of hierarchical accountability mechanisms on their own. Horizontal mechanisms for accountability have been developed in which agencies account for their behavior toward stakeholders, boards of commissioners, and professional visitations. This article addresses the question whether and to what extent horizontal accountability mechanisms in practice serve as remedies for the deficit. An evaluative grading system is designed with which 15 horizontal accountability mechanisms of Dutch agencies are assessed. The analysis shows that horizontal accountability is a partial solution for the accountability deficit. It scarcely contributes to democratic control but has promising effects for organizational learning.