  We demonstrate theoretically that most of the observed transport properties
of graphene sheets at zero magnetic field can be explained by scattering from
charged impurities. We find that, contrary to common perception, these
properties are not universal but depend on the concentration of charged
impurities $n_{\rm imp}$. For dirty samples ($250 \times 10^{10} {\rm cm}^{-2}
< n_{\rm imp} < 400 \times 10^{10} {\rm cm}^{-2}$), the value of the minimum
conductivity at low carrier density is indeed $4 e^2/h$ in agreement with early
experiments, with weak dependence on impurity concentration. For cleaner
samples, we predict that the minimum conductivity depends strongly on $n_{\rm
imp}$, increasing to $8 e^2/h$ for $n_{\rm imp} \sim 20 \times 10^{10}{\rm
cm}^{-2}$. A clear strategy to improve graphene mobility is to eliminate
charged impurities or use a substrate with a larger dielectric constant.
