Plants produce lateral roots (LRs) throughout their life cycle to continually expand their root system. The formation and growth of lateral roots are important for plant nutrient and water acquisition and anchorage. Despite their importance, the dynamic process of lateral root development remains elusive. Here, we used 3-D live imaging of lateral root growth in the model plant Arabidopsis thaliana to reveal how lateral roots emerge from mother roots and penetrate surrounding tissues. Our results reveal three stages of lateral root development: initiation, emergence, and elongation, each characterized by distinct cellular behaviors. We uncovered that the emergence of the lateral root primordium from the mother root is a two-step process involving localized cell divisions at the proximal and distal regions of the primordium. This work provides a comprehensive view of lateral root development and insight into the cellular and molecular mechanisms that control this process.