  This chapter is in three parts. The first presents the fundamental equations
of level populations and radiative transfer which govern spectral line emission
and absorption. Lines in the cm-wave band have similarities which allow us to
simplify the equations. The further simplification of a two level system (a
good approximation for H I gives the familiar formulae for the 21-cm line.
  The second part of the lesson deals with mapping, and the combination of
single dish and interferometer data. We consider the effect of gridding single
dish data using a convolving function. Mapping speeds and techniques are
discussed.
  The third part discusses the spectral line cube and its moments. The use of
the first moments to determine the dynamical structure of a disk is discussed.
