On invoking XSTAR from the command line, the user is prompted for a series of input values which are described below. These parameters are stored in a parameter file xstar.par, which must live in the directory specified by the user's PFILES environment variable. Upon the initial invocation of XSTAR, the default version of the parameter file is copied into the pfiles directory. Each parameter has associated with it a prompt string and a parameter name. The former is used when values are input via prompting, and the latter when values are input via the command line. In sections 1 - 3 we describe the parameters used for most straightforward applications of XSTAR. In section 5 we discuss the parameters which are ordinarily hidden from prompting, and which provide added flexibility.