Abstract: Globally, freshwater ecosystems harbour a disproportionately high biodiversity, yet rates of loss are higher than for those in the terrestrial or marine environment. In Europe, a major reason why member states fail to meet Water Framework Directive requirements to ensure water bodies achieve good ecological status is due to the impact of a high density of river infrastructure, the legacy of a long history of river engineering. Fish passes are widely installed to mitigate for the environmental impact of barriers, yet efficiency is rarely evaluated. Today the need for robust and standardised methods to evaluate fish passes is recognised. This requires identification of areas where understanding is limited, and the development of a coherent programme of fish pass evaluation so that knowledge gaps may be closed. Coherent field evaluation (telemetry) studies should quantify delay and the efficiency of: a) attraction, b) entrance, and c) passage. Comparisons of efficiencies obtained against predefined goals should be biologically relevant, focusing on values expected to maintain sustainable populations taking into account cumulative impacts of barrier networks. Achieving this would represent a “great leap forward”; but is only half the challenge. It is likely that a standardised evaluation will highlight the high degree of variability in efficiency and that most fish passes perform at levels below expectation. Thus, there is an equally important need to understand why fish passes provide only half-way technologies and partial solutions, and in some cases prove to be ecologically damaging. The main focus of future research should be to improve fish pass design through collaborative and holistic experimental studies that return to fundamental first principles to enhance understanding of fish performance (e.g. swimming, leaping, climbing) and behaviour (e.g. motivation, response to hydrodynamics, acoustics and other stimuli), the results of which are validated in the field.