Plant development is in many aspects unique and is characterized by a continuous growth and flexible adjustments of the plant architecture in response to the environment. To achieve its profound adaptability, plants are able to maintain permanent stem cell populations, dedifferentiate already committed cells and, not least, to regenerate or form organs de novo. These developmental processes are governed and coordinated by signalling substances called hormones such as auxin, abscisic acid, brassinosteroid, cytokinin, ethylene, gibberellin, and jasmonic acid. Over the past decades, physiological and genetic studies have revealed that hormone action in plants is determined by complex interactions between hormonal signalling pathways. To identify molecular components of hormonal metabolism and signalling, genetic approaches have been successfully used and led to a basic molecular understanding of hormone action in plants. However, molecular basis for hormonal cross-talk is still largely unknown and its clarification represents a major challenge in the coming years for plant biology research. We use lateral root formation in Arabidopsis as a model system to study mechanisms of hormonal interactions regulating plant organogenesis.