Large-scale exploratory approaches to understanding gene function laid the foundations for the “-omics era”. Based on modern technologies for the structural and functional characterization of genomes, these curiosity-driven approaches allow systematic accumulation of vast amounts of data, enabling subsequent hypothesis-driven research. Some years before the dawn of genomics, exploratory approaches were already furthering our understanding of gene function in the form of saturation mutagenesis experiments aimed at the identification of all genes that mutate to a given phenotype. Forward genetic approaches, conducted on experimental organisms such as Drosophila melanogaster and Caenorhabditis elegans, have led to the isolation of mutants affected in specific developmental processes, whose cellular and molecular characterization has unraveled the underlying genetic mechanisms of animal development. To shed light on the making of plant leaves, in 1993 we initiated an attempt to identify as many viable and fertile mutants with abnormal leaf morphology as possible, using the Arabidopsis thaliana model organism. We identified 25 fast-neutron- and 153 ethyl-methane sulfonate-induced mutations, which fell into eight and 94 complementation groups, respectively. We also studied 115 publicly available mutant lines isolated by previous authors, which fell into 37 complementation groups. Although we did not reach saturation of the Arabidopsis thaliana genome, the broad spectrum of leaf morphological alterations identified is facilitating the dissection of specific leaf developmental processes. In a complementary approach, we also analyzed leaf architecture in natural accessions and two populations of recombinant inbred lines. Using a high-throughput gene mapping method, we have already cloned 25 of the genes identified by mutation, in some cases in collaboration with other groups. The products of these genes participate in various developmental processes, such as polar cell expansion, transduction of hormonal signals, gene regulation, plastid biogenesis, and chromatin remodeling, among others. The range of phenotypes and processes identified reveal the complexity of leaf ontogeny and will help explain the diversity of leaf morphology in nature.