Kelp forests are threatened by human activities that result in habitat loss or deplete natural stocks, but little is known about genetic diversity, importance of gene flow and effect of population fragmentation on genetic structure. We reviewed the literature to characterize the patterns of connectivity and scales of population structure in kelps. In all, 44 papers have been published on the patterns of genetic differentiation in 17 kelp species, using various kinds of molecular markers. Our literature review showed that population connectivity depends mainly on species' dispersal abilities and habitat characteristics (intertidal vs subtidal), but little on their life-span characteristics. Data on within-population gene diversity were found for only seven species and reflect differences in effective population sizes. In addition, we focused on the Brittany populations of Laminaria digitata, one of the most commonly harvested species in Europe, and re-sampled populations studied seven years prior. Our analyses of spatio-temporal variation clearly demonstrate the effect of small population sizes on the genetic instability of isolated L. digitata populations and have implications for managing this genetic resource.