  Beams of polar molecules can be focused using an array of electrostatic
lenses in alternating gradient (AG) configuration. They can also be accelerated
or decelerated by applying an appropriate high voltage switching sequence to
the lenses. AG focusing is applicable to molecules in both low-field and
high-field-seeking states and is particularly well suited to the problem of
decelerating heavy molecules and those in their ground rotational state. We
describe the principles of AG deceleration and set out criteria to be followed
in decelerator design, construction and operation. We calculate the
longitudinal and transverse focusing properties of a decelerator, and exemplify
this by 2D-imaging studies of a decelerated beam of metastable CO molecules.
