Classical morphological studies of plant development have provided a sound basis for recent advances in molecular and computational tools for understanding how leaves become leaves. Research on mutants in model organisms has identified networks of genes that are involved in the development of leaves. Mutant analyses and gene expression studies have also revealed epigenetic phenomena that regulate leaf development. Such techniques are still invaluable to developmental biology, and these techniques are continuing to be enhanced. Each new technique or species studied appears to reveal further levels of complexity in the regulation of leaf development. Only by drawing together evidence from numerous techniques and comparative studies of a wide array of species will we understand how such diversity of plant form has arisen. In this article we do not provide a comprehensive review of our current understanding of leaf development, but rather a glimpse at how advances in technologies facilitated that understanding.