We discuss the rotational excitation of small interstellar grains and the r
esulting electric dipole radiation from spinning dust. Attention is given t
o excitation and damping of grain rotation by collisions with neutrals, col
lisions with ions, "plasma drag," emission of infrared radiation, emission
of electric dipole radiation, photoelectric emission, and formation of H-2
on the grain surface. Electrostatic "focusing" can substantially enhance th
e rate of rotational excitation of grains colliding with ions. Under some c
onditions, "plasma drag"-due to interaction of the electric dipole moment o
f the grain with the electric field produced by passing ions-dominates both
rotational damping and rotational excitation. Emissivities are estimated f
or dust in different phases of the interstellar medium, including diffuse H
I clouds, warm H I, low-density photoionized gas, and cold molecular gas.
Spinning dust grains could explain much, and perhaps all, of the 14-50 GHz
background component recently observed by Kogut et al., de Oliveira-Costa e
t al., and Leitch et aI. Future sensitive measurements of angular structure
in the microwave sky brightness from the ground and from space should dete
ct this emission from high-latitude H I clouds. It should be possible to de
tect rotational emission from small grains by groundbased pointed observati
ons of molecular clouds, unless these grains are less abundant there than i
s currently believed.