Over the past twenty-five years, observations of the Cosmic Microwave Background (CMB) temperature fluctuations have served as an important tool for answering some of the most fundamental questions of modern cosmology: how did the universe begin, what is it made of, and how did it evolve? More recently, measurements of the faint polarization signatures of the CMB have offered a complementary means of probing these questions, helping to shed light on the mysteries of cosmic inflation, relic neutrinos, and the nature of dark energy. A second-generation receiver for the Atacama Cosmology Telescope (ACT), the Atacama Cosmology Telescope Polarimeter (ACTPol), was designed and built to take advantage of both these cosmic signals by measuring the CMB to high precision in both temperature and polarization. The receiver features three independent sets of cryogenically cooled optics coupled to transition-edge sensor (TES) based polarimeter arrays via monolithic silicon feedhorn stacks. The three detector arrays, two operating at 149 GHz and one operating at both 97 and 149 GHz, contain over 1000 detectors each and are continuously cooled to a temperature near 100 mK by a custom-designed dilution refrigerator insert. Using ACT's six meter diameter primary mirror and diffraction limited optics, ACTPol is able to make high-fidelity measurements of the CMB at small angular scales (l ~ 9000), providing an excellent complement to Planck. The design and operation of the instrument are discussed in detail, and results from the first two years of observations are presented. The data are broadly consistent with /\CDM and help improve constraints on model extensions when combined with temperature measurements from Planck.