The Siluro-Devonian fossil record provides unequivocal evidence for independent origins of leaves in lycophytes and euphyllophytes, and in several lineages among the latter.  In each lineage, leaf-bearing members are stratigraphically younger and more derived, compared to those with plesiomorphic organization lacking stem-leaf differentiation.  However, the evolutionary processes underpinning each independent leaf origin have remained largely unresolved.  Lycophyte leaves arose in the earliest Devonian (possibly latest Silurian).  Among euphyllophytes, leaves appear 30 million-years later, toward the end of the Middle Devonian.  Evolving under the same physiological and developmental-structural constraints– maximizing photosynthetic yield and arising as appendages from a meristem required to maintain indeterminacy –, leaves have converged toward the same defining features: vascularization, bilateral symmetry and adaxial-abaxial polarity, regular arrangement, and determinate growth.  These structural features are underpinned by regulatory mechanisms such as auxin-mediated primordium positioning (regular arrangement), KNOX/ARP interactions (leaf determinacy), and HD-ZIPIII/KANADI interactions (adaxial-abaxial polarity).  Leaf evolution can be addressed by deconstructing leaves into their defining structural features – that can be observed in living plants and traced back in the fossil record – and developmental regulatory mechanisms – that can be studied in living representatives of different lineages.  Lycophyte leaves likely have a single origin, having evolved either by vascularization of enations or by sterilization of sporangia.  Consistent with an independent origin, lycophyte leaves seem to lack the adaxial-abaxial patterning function that HD-ZIPIII/KANADI interactions have in euphyllophytes.  The ancestral role of this regulatory pathway in the differentiation of sporangia and vascular tissue is more consistent with the sporangium sterilization hypothesis.  Given the scarcity of anatomically-preserved fossils in the key interval for lycophyte leaf evolution (Silurian – earliest Devonian), understanding lycophyte leaf origins will require new and substantial fossil discoveries.  The traditional view that euphyllophyte leaves evolved from lateral branching systems has supported a single origin of leaves in the clade.  However, developmental regulation data show differences between ferns and seed plants in the deployment and roles of KNOX/ARP and HDZIPIII/KANADI pathways, supporting multiple origins.  Additionally, the better fossil coverage of the Early-Middle Devonian interval renders the fossil record more tractable for addressing euphyllophyte leaf origins.  A close look at the lateral branching systems of Early-Middle Devonian euphyllophytes shows a breadth of structural diversity that belies their apparent simplicity and morphological similarity, supports multiple leaf origins, and may offer clues on tempo and mode of leaf evolution in the different lineages.  By the end of the Early Devonian, at least two euphyllophyte lineages had evolved lateral appendages with regular arrangement and possessing (one of them) bilateral symmetry.  A major hurdle is the lack of phylogenetic connection between these Early Devonian plants and Middle and Late Devonian plants in which we recognize bona fide leaves.  Better resolved phylogenies of Early through Late Devonian plants are needed to provide a framework for tracing the appearance of different leaf-defining features in each lineage.  Significant advances will also come from functional studies in different extant seed-free lineages where such information is currently minimal, to understand the developmental mechanisms of parallel evolution of leaves.