Research effort is unevenly distributed across species, which can cause important biases in our understanding of evolutionary and ecological processes and affect conservation decisions. For example, many shark species remain understudied, despite the fact that sharks play fundamental roles in marine ecosystems and are particularly affected by fisheries. Assessing and acknowledging these differences in research effort across shark species is a key step to improving our knowledge and management of shark populations as it allows researchers to both target species in need of research and control for potential biases when performing comparative analyses. I provide here an index of research effort (the number of articles published between 1978 and 2014 listed in the Zoological Record database) for 509 shark species that can be downloaded and utilized in future comparative analyses. I then show that research effort is not randomly distributed across species within the clade, but is significantly predicted by taxonomy, geography, ecology, life history and extinction risk. For example, large species that occur across a large range of latitudes and at shallow minimum depths have attracted considerably more research. The consequence is a strong bias in our knowledge of sharks towards species that are not an unbiased representative sample of the clade. The database and the patterns demonstrated here call for more awareness of the biases in research effort and their potential consequences, not only for practical considerations such as the interpretation of the results of comparative analyses and the assessment of extinction risk, but also for our basic understanding of the ecological roles of sharks.