The complex and adaptable architecture of the plant root system in soil is of paramount importance for crop growth and performance. Root growth depends on the activity of the root apical meristem, an organized population of proliferating progenitor cells continuously replenished from a stem cell niche. Root branching, which greatly contributes to root system architecture in most dicot species, consists in de novo formation of new root meristems in existing root tissues. This phenomenon illustrates the ability of plants to repeatedly generate new tissues specialized in post-embryonic continuous growth and greatly impacts the elaboration of the root system architecture and its adaptation to environmental constraints. Here, we review the recent findings and models related to lateral root organogenesis in the dicot species Arabidopsis 2 thaliana, with emphasis on the mechanisms controlling de novo root meristem formation.Experimental evidence suggests that critical regulatory modules are common between embryonic and post-embryonic root meristem organogenesis, and that the lateral root formation molecular pathway is in part common with organ regeneration from callus.