The WMAP 5-year data provide stringent limits on deviations from the minimal 6-parameter ΛCDM model. These limits are used to constrain cosmic inflation through Gaussianity, adiabaticity, the power spectrum of primordial fluctuations, gravitational waves, and spatial curvature. The data also constrain dark energy models via its equation of state, parity-violating interactions, and neutrino properties. No convincing deviations from the minimal model are detected. The 6 parameters and their 68% uncertainties, derived from WMAP data combined with Type Ia supernovae (SN) and Baryon Acoustic Oscillations (BAO) measurements, are: Ω_bh² = 0.02267 ± 0.00059, Ω_ch² = 0.1131 ± 0.0034, Ω_Λ = 0.726 ± 0.015, n_s = 0.960 ± 0.013, τ = 0.084 ± 0.016, and Δ_R² = (2.445 ± 0.096) × 10⁻⁹ at k = 0.002 Mpc⁻¹. From these, σ₈ = 0.812 ± 0.026, H₀ = 70.5 ± 1.3 km s⁻¹ Mpc⁻¹, Ω_b = 0.0456 ± 0.0015, Ω_c = 0.228 ± 0.013, Ω_mh² = 0.1358 ± 0.0037, z_reion = 10.9 ± 1.4, and t₀ = 13.72 ± 0.12 Gyr are derived. The tensor-to-scalar ratio r < 0.22 (95% CL) and n_s > 1 is disfavored, constraining inflation models that produce significant gravitational waves. Tight limits on the dark energy equation of state and spatial curvature are found: -0.14 < 1 + w < 0.12 (95% CL) and -0.0179 < Ω_k < 0.0081 (95% CL). WMAP distance priors are provided to test dark energy models with spatial curvature. A time-dependent w with w₀ constrained as -0.33 < 1 + w₀ < 0.21 (95% CL) is tested. Temperature and dark matter fluctuations obey the adiabatic relation to within 8.9% and 2.1% for axion-type and curThe WMAP 5-year data provide stringent limits on deviations from the minimal 6-parameter ΛCDM model. These limits are used to constrain cosmic inflation through Gaussianity, adiabaticity, the power spectrum of primordial fluctuations, gravitational waves, and spatial curvature. The data also constrain dark energy models via its equation of state, parity-violating interactions, and neutrino properties. No convincing deviations from the minimal model are detected. The 6 parameters and their 68% uncertainties, derived from WMAP data combined with Type Ia supernovae (SN) and Baryon Acoustic Oscillations (BAO) measurements, are: Ω_bh² = 0.02267 ± 0.00059, Ω_ch² = 0.1131 ± 0.0034, Ω_Λ = 0.726 ± 0.015, n_s = 0.960 ± 0.013, τ = 0.084 ± 0.016, and Δ_R² = (2.445 ± 0.096) × 10⁻⁹ at k = 0.002 Mpc⁻¹. From these, σ₈ = 0.812 ± 0.026, H₀ = 70.5 ± 1.3 km s⁻¹ Mpc⁻¹, Ω_b = 0.0456 ± 0.0015, Ω_c = 0.228 ± 0.013, Ω_mh² = 0.1358 ± 0.0037, z_reion = 10.9 ± 1.4, and t₀ = 13.72 ± 0.12 Gyr are derived. The tensor-to-scalar ratio r < 0.22 (95% CL) and n_s > 1 is disfavored, constraining inflation models that produce significant gravitational waves. Tight limits on the dark energy equation of state and spatial curvature are found: -0.14 < 1 + w < 0.12 (95% CL) and -0.0179 < Ω_k < 0.0081 (95% CL). WMAP distance priors are provided to test dark energy models with spatial curvature. A time-dependent w with w₀ constrained as -0.33 < 1 + w₀ < 0.21 (95% CL) is tested. Temperature and dark matter fluctuations obey the adiabatic relation to within 8.9% and 2.1% for axion-type and cur