The Mathematical Modeling Laboratory (MLAB), described in this PRIMER, was developed as a tool for experimentation with, and evaluation of, mathematical models or functions. It runs on a DEC SYSTEM 10 or 20 timesharing system and is available for public distribution. Chapters 2 and 3 deal with curve fitting and differential equations and proceed from elementary examples to the more complex. Chapter 4 introduces a few of the special commands in MLAB: the ROOT operator, the SUM command and the FOR command. These are basic commands and do not depend on anything in Chapters 2 and 3. Chapter 5 contains a discussion of files and in particular a description of DO FILES. DO FILES are the mechanism within MLAB to develop a programming language; that is, a sequence of executable commands which can be called in as a package to operate on data. Chapter 6 is essentially comprised of detailed examples illustrating the contents of the first five chapters.