The six-meter Atacama Cosmology Telescope (ACT) in Chile was built to measure the cosmic microwave background
(CMB) at arcminute angular scales. We are building a new polarization sensitive receiver for ACT
(ACTPol). ACTPol will characterize the gravitational lensing of the CMB and aims to constrain the sum of the
neutrino masses with ~ 0.05 eV precision, the running of the spectral index of inflation-induced fluctuations,
and the primordial helium abundance to better than 1 %. Our observing fields will overlap with the SDSS BOSS
survey at optical wavelengths, enabling a variety of cross-correlation science, including studies of the growth of
cosmic structure from Sunyaev-Zel'dovich observations of clusters of galaxies as well as independent constraints
on the sum of the neutrino masses. We describe the science objectives and the initial receiver design.