  We treat free large N superconformal field theories as holographic duals of
higher spin (HS) gauge theories expanded around AdS spacetime with radius R.
The HS gauge theories contain massless and light massive AdS fields. The HS
current correlators are written in a crossing symmetric form including only
exchange of other HS currents. This and other arguments point to the existence
of a consistent truncation to massless HS fields. A survey of massless HS
theories with 32 supersymmetries in D=4,5,7 (where the 7D results are new) is
given and the corresponding composite operators are discussed. In the case of
AdS_4, the cubic couplings of a minimal bosonic massless HS gauge theory are
described. We examine high energy/small tension limits giving rise to massless
HS fields in the Type IIB string on AdS_5 x S^5 and M theory on AdS_{4/7} x
S^{7/4}. We discuss breaking of HS symmetries to the symmetries of ordinary
supergravity, and a particularly natural Higgs mechanism in AdS_5 x S^5 and
AdS_4 x S^7 where the HS symmetry is broken by finite g_{YM}. In AdS_5 x S^5 it
is shown that the supermultiplets of the leading Regge trajectory cross over
into the massless HS spectrum. We propose that g_{YM}=0 corresponds to a
critical string tension of order 1/R^2 and a finite string coupling of order
1/N. In AdS_7 x S^4 we give a rotating membrane solution coupling to the
massless HS currents, and describe these as limits of Wilson surfaces in the
A_{N-1}(2,0) SCFT, expandable in terms of operators with anomalous dimensions
that are asymptotically small for large spin. The minimal energy configurations
have semi-classical energy E=s for all s and the geometry of infinitely
stretched strings with energy and spin density concentrated at the endpoints.
