  Measurement of the parity-violating electron scattering asymmetry from
${}^{208}$Pb has demonstrated a new opportunity at Jefferson Lab to measure the
weak charge distribution and hence pin down the neutron radius in nuclei in a
relatively clean and model-independent way. This is because the Z boson of the
weak interaction couples primarily to neutrons. We will describe the PREX and
CREX experiments on ${}^{208}$Pb and ${}^{48}$Ca respectively. PREX-I ran in
2010, and CREX and a second run of PREX are currently in preparation.
