This review attempts to assess the nature and the role of cover for riverine fish assemblages. Although early identified as a key factor for fish distribution, especially for salmonids, cover (i.e. woody debris, undercut banks, boulders, turbidity...) still remains the variable least considered in the studies of fish habitat relationships. This is mainly due to the diversity of ecological functions of cover structures in fish assemblages. Cover structures are structuring components of fish habitat and contribute to the biological productivity of streams. But, at the individual scale, cover fulfils three main functions: protection against predators, visual isolation reducing competition, and hydraulic shelter. In fact, the use of cover by fish results from a trade-off between the costs and the benefits associated with its use. Although the relationships between fish and cover appear extremely complex and context-specific, a growing body of evidence highlights the potential role of cover for management purposes.