We present cosmological parameters derived from the angular power spectrum of the cosmic microwave background ( CMB ) radiation observed at 148 GHz and 218 GHz over 296 deg ^ { 2 } with the Atacama Cosmology Telescope ( ACT ) during its 2008 season . ACT measures fluctuations at scales 500 < \ell < 10000 . We fit a model for the lensed CMB , Sunyaev-Zel ’ dovich ( SZ ) , and foreground contribution to the 148 GHz and 218 GHz power spectra , including thermal and kinetic SZ , Poisson power from radio and infrared point sources , and clustered power from infrared point sources . At \ell = 3000 , about half the power at 148 GHz comes from primary CMB after masking bright radio sources . The power from thermal and kinetic SZ is estimated to be { \cal B } _ { 3000 } = 6.8 \pm 2.9 ~ { } \mu { K } ^ { 2 } , where { \cal B } _ { \ell } \equiv \ell ( \ell + 1 ) { C } _ { \ell } / 2 \pi . The IR Poisson power at 148 GHz is { \cal B } _ { 3000 } = 7.8 \pm 0.7 ~ { } \mu { K } ^ { 2 } ( C _ { \ell } = 5.5 \pm 0.5 ~ { } { nK } ^ { 2 } ) , and a clustered IR component is required with { \cal B } _ { 3000 } = 4.6 \pm 0.9 ~ { } \mu { K } ^ { 2 } , assuming an analytic model for its power spectrum shape . At 218 GHz only about 15 % of the power , approximately 27 ~ { } \mu { K } ^ { 2 } , is CMB anisotropy at \ell = 3000 . The remaining 85 % is attributed to IR sources ( approximately 50 % Poisson and 35 % clustered ) , with spectral index \alpha = 3.69 \pm 0.14 for flux scaling as S ( \nu ) \propto \nu ^ { \alpha } . We estimate primary cosmological parameters from the less contaminated 148 GHz spectrum , marginalizing over SZ and source power . The \Lambda CDM cosmological model is a good fit to the data ( \chi ^ { 2 } / { dof } = 29 / 46 ) , and \Lambda CDM parameters estimated from ACT+ WMAP are consistent with the 7-year WMAP limits , with scale invariant n _ { s } = 1 excluded at 99.7 % CL ( 3 \sigma ) . A model with no CMB lensing is disfavored at 2.8 \sigma . By measuring the third to seventh acoustic peaks , and probing the Silk damping regime , the ACT data improve limits on cosmological parameters that affect the small-scale CMB power . The ACT data combined with WMAP give a 6 \sigma detection of primordial helium , with Y _ { P } = 0.313 \pm 0.044 , and a 4 \sigma detection of relativistic species , assumed to be neutrinos , with N _ { eff } = 5.3 \pm 1.3 ( 4.6 \pm 0.8 with BAO+ H _ { 0 } data ) . From the CMB alone the running of the spectral index is constrained to be dn _ { s } / d \ln k = -0.034 \pm 0.018 , the limit on the tensor-to-scalar ratio is r < 0.25 ( 95 % CL ) , and the possible contribution of Nambu cosmic strings to the power spectrum is constrained to string tension G \mu < 1.6 \times 10 ^ { -7 } ( 95 % CL ) .