Roots are major appendages of the plant used for anchorage and for water and nutrient uptake. The roles of roots are numerous, and they constitute a major pathway for the transfer of water from soil into the atmosphere. Root growth is normally genetically controlled, but a host of other factors such as the soil and the plant–plant interactions affect the overall development and distribution patterns. This chapter briefly reviews the knowledge of root growth in mixed plant systems, focusing on how root–soil interactions influence the growth processes. A conceptual framework for simulating the growth and distribution of the component plant root systems is presented, which is based on factors such as compaction, soil water distribution, root carrying capacity of different soil layers, and interspecies interactions. Though still lacking in some respects, the concept was able to simulate a range of observed root growth and water uptake patterns in a realistic manner. The proposed framework does provide a basis for further development of plant growth modeling in mixed systems.