This bulletin 178, Operation of Hydraulic Structures of Dams, is an update of Bulletin 49A (1986), which was the second edition of Bulletin 49 (1984). The current update was prepared using developments and progress made in the last 30 years with operation equipment, staff building and training, and regulatory requirements. Bulletin 178 addresses the need for safe reservoir discharge under a variety of conditions, the dam operator’s staffing, evaluation (inspection) of the condition of operating equipment, and operation during unusual or extreme conditions. The operation during unusual or extreme conditions is generally focused on flood and the current abilities to predict significant precipitation events, monitor the flood approach and impact, and communicate and implement the actions needed for safe operation. An annex is provided with seven case studies that provide relevant histories for the subject matter.