  We review some recent work on AdS/CFT duality involving the 3d O(N) Vector
Model and AdS4 Higher Spin Gravity. Our construction is based on bi-local
collective field theory which provides an off-shell formulation of Higher Spin
Gravity with G = 1/N playing the role of a coupling constant. Consequently
perturbative and non-perturbative issues of the theory can be studied. For the
correspondence based on free CFT's we discuss the nature of bulk 1/N
interactions through an S-matrix which is argued to be equal to 1
(Coleman-Mandula theorem). As a consequence in this class of theories
nonlinearities are removable, through a nonlinear field transformation which we
show at the exact level. We also describe a geometric (Kahler space) framework
for the bi-local theory which applies equally simple to Sp(2N) fermions and the
de Sitter correspondence. We discuss in this framework the structure and size
of the bi-local Hilbert space and the implementation of (finite N) exclusion
principle.
