Auxins are plant hormones involved in many different plant developmental processes. To understand the biosynthesis of auxins is one step towards understanding the regulation of these hormones during plant development. The main auxin in plants, indole-3-acetic acid (IAA), can be synthesized via different pathways and from different precursors. Multiple pathways may exist within the plant species in different tissues or during different developmental processes. The possible routes for IAA biosynthesis will be summarized and arguments for and against the different pathways will be presented. Another naturally occurring auxin is indole-3-butyric acid (IBA). Although IBA is a very prominent auxin in horticultural practice, it was only recently established that it is a native compound in various species such as maize (Zea mays L.) and Arabidopsis thaliana. The biosynthesis of IBA was first investigated in maize and it was found that IAA is converted to IBA in vivo. The enzyme system which catalyzes the reaction was characterized and will be described in detail. A possible role for IBA during plant development will be discussed.