Feeding inside leaves by leaf-mining insects has been viewed as an adaptive innovation for consuming foliage. The leaf mine presumably provides its occupant shelter from the detrimental effects of the physical environment, protection from attack by natural enemies, and potentially a means of avoiding plant defenses concentrated in particular layers of leaf tissue. We examine several hypotheses concerning the adaptive significance of leaf mining using both the published literature and experiments. We also examine the notion that leaf mining is an innovation that has led to an adaptive radiation by examining the phylogeny and species richness of leaf-mining lineages and their sister groups. Both advantages and disadvantages accrue to insects that feed inside leaf mines in comparison to external-feeding folivores. The advantages of feeding within a leaf mine include: a lower incidence of disease infection, a microenvironment with lower evaporative demand and therefore protection from desiccation, protection from the direct and indirect effects of UV radiation by the leaf-mine epidermis, and avoidance of plant defenses resulting in higher feeding efficiencies. The disadvantages of feeding inside a leaf mine include: lower mobility resulting in higher mortality from parasites, higher mortality associated with premature leaf-abscission, and smaller average body size and lower fecundity. Leaf mining is certainly a viable means of feeding upon foliage, and in particular instances it might be the most successful means of feeding on leaves. However, the lower species richness of leaf-mining lineages in comparison to their external-feeding sister groups indicates that the evolution of leaf mining does not represent an innovation that has led to an adaptive radiation. Perhaps the evolution of leaf-feeding per se and the evolution of the ability to feed externally on leaves rather than concealed feeding modes such and mining and galling represent innovations that have resulted in adaptive radiations in the Insecta.