We present 197 planet candidates discovered using data from the first year of the NASA K2 mission (Campaigns 0-4), along with the results of an intensive program of photometric analyses, stellar spectroscopy, high-resolution imaging, and statistical validation. We distill these candidates into sets of 104 validated planets (57 in multi-planet systems), 30 false positives, and 63 remaining candidates. Our validated systems span a range of properties, with median values of R_P_=2.3R_{Earth}, P=8.6 days, Teff=5300K, and Kp=12.7mag. Stellar spectroscopy provides precise stellar and planetary parameters for most of these systems. We show that K2 has increased by 30% the number of small planets known to orbit moderately bright stars (1-4R{Earth}, Kp=9-13mag). Of particular interest are 76 planets smaller than 2R{Earth}, 15 orbiting stars brighter than Kp=11.5mag, 5 receiving Earth-like irradiation levels, and several multi-planet systems-including 4 planets orbiting the M dwarf K2-72 near mean-motion resonances. By quantifying the likelihood that each candidate is a planet we demonstrate that our candidate sample has an overall false positive rate of 15%-30%, with rates substantially lower for small candidates (8R{Earth}_ and/or with P<3days. Extrapolation of the current planetary yield suggests that K2 will discover between 500 and 1000 planets in its planned four-year mission, assuming sufficient follow-up resources are available. Efficient observing and analysis, together with an organized and coherent follow-up strategy, are essential for maximizing the efficacy of planet-validation efforts for K2, TESS, and future large-scale surveys.

Cone search capability for table J/ApJS/226/7/table10 (High-resolution imaging)

Cone search capability for table J/ApJS/226/7/table3 (HIRES follow-up observations)

Cone search capability for table J/ApJS/226/7/table4 (FEROS radial velocities)

Cone search capability for table J/ApJS/226/7/table5 (HRI-detected stellar companions)

Cone search capability for table J/ApJS/226/7/table7 (Stellar parameters)