  We explain how the string spectrum in flat space and pp-waves arises from the
large $N$ limit, at fixed $g^2_{YM}$, of U(N) ${\cal N} =4$ super Yang Mills.
We reproduce the spectrum by summing a subset of the planar Feynman diagrams.
We give a heuristic argument for why we can neglect other diagrams. We also
discuss some other aspects of pp-waves and we present a matrix model associated
to the DLCQ description of the maximally supersymmetric eleven dimensional
pp-waves.
