Interactive Data Language (IDL) is a complete data analysis and visualization environment that is used in a wide range of science and engineering disciplines for processing and analyzing numerical and image data. It is often used in advanced science/technical courses. Professor Ken Bowman originally developed this text for the laboratory portion of an undergraduate course on Physical Climatology, but his emphasis on fundamental concepts and practical topics helps students write programs for other classes or for their research. This primer is aimed at beginning programmers, not experienced C or Fortran programmers who are new to IDL. *Lucid writing style*End-of-chapter summaries*End-of-chapter exercises