  We show that the large $N$ limit of certain conformal field theories in
various dimensions include in their Hilbert space a sector describing
supergravity on the product of Anti-deSitter spacetimes, spheres and other
compact manifolds. This is shown by taking some branes in the full M/string
theory and then taking a low energy limit where the field theory on the brane
decouples from the bulk. We observe that, in this limit, we can still trust the
near horizon geometry for large $N$. The enhanced supersymmetries of the near
horizon geometry correspond to the extra supersymmetry generators present in
the superconformal group (as opposed to just the super-Poincare group). The 't
Hooft limit of 4-d ${\cal N} =4$ super-Yang-Mills at the conformal point is
shown to contain strings: they are IIB strings. We conjecture that
compactifications of M/string theory on various Anti-deSitter spacetimes are
dual to various conformal field theories. This leads to a new proposal for a
definition of M-theory which could be extended to include five non-compact
dimensions.
