The section on the left side of the main GSAS-II has what is called the GSAS-II data tree, which shows a hierarchical view of information stored inside a GSAS-II project, largely mirroring the way that information is stored inside a GSAS-II project file. Since there is considerable flexibility with regard to what information may be stored inside a project, there can also be considerable variability as to the types of entries that will be seen in the data tree. GSAS-II is operated by selecting different data tree items and changing the information associated with them. This chapter outlines the entries that can be found in the data tree and what sort of information is stored in each.