The kernel needs numerous data structures to administer and display the network. The kernel has to display the units, links and sites (the static component), and to save the functions and connection weights of the units (the dynamic component). A symbol table is used to handle all the names and types associated with the units which are defined by the user. The allocation of data structures is done in large blocks of several hundred or even thousands of single structure components. This chapter describes all the relevant data types in detail.