HD-Zip proteins are unique to plants, and contain a homeodomain closely linked to a leucine zipper motif, which are involved in dimerization and DNA binding. Based on homology in the HD-Zip domain, gene structure and the presence of additional motifs, HD-Zips are divided into four families, HD-Zip I–IV. Phylogenetic and bioinformatics analysis of HD-Zip genes using transcriptomic and genomic datasets from a wide range of plant species indicate that the HD-Zip protein class was already present in green algae. Later, HD-Zips experienced multiple duplication events that promoted neo- and sub-functionalizations. HD-Zip proteins are known to control key developmental and environmental responses, and a growing body of evidence indicates a strict link between members of the HD-Zip II and III families and the auxin machineries. Interactions of HD-Zip proteins with other hormones such as brassinolide and cytokinin have also been described. However, it is striking that among the genes regulated by REV, a HD-Zip III protein playing a key role in apical development, are genes that mediate ABA signaling. Furthermore, HAT1 and HAT3, two HD-Zip II proteins involved in key developmental processes, repress ABA biosynthesis and signaling, indicating an essential role of these factors in adjusting development to changing environment.