  We propose and quantize a local, covariant gauge-field action that unifies
the description of all free helicity and continuous-spin degrees of freedom in
a simple manner. This is the first field-theory action of any kind for
continuous spin particles; it is consistent as a quantum theory and generalizes
to any number of dimensions. The fields live on the null cone of an internal
four-vector "spin-space"; in D dimensions a linearized gauge invariance reduces
their physical content to a single function on a Euclidean (D-2)-plane, on
which the little group E(D-2) acts naturally. A projective version of the
action further reduces the physical content to S^{D-3}, enabling a new local
description of particles with any spin structure, and in particular a tower of
all integer-helicity particles for D=4. Gauge-invariant interactions with a
background current are added in a straightforward manner.
