The paper by D. R. Smith and S. Schultz, along with P. Markos and C. M. Soukoulis, analyzes the reflection and transmission coefficients of electromagnetic metamaterials to determine their effective permittivity (ε) and permeability (μ). The authors use transfer matrix simulations on finite lengths of metamaterials, including periodic arrangements of wires, split ring resonators (SRRs), and a combination of both. They find that the recovered frequency-dependent ε and μ are consistent with analytic expressions predicted by effective medium arguments. Notably, a wire medium exhibits a frequency region with a negative real part of ε, and SRRs produce a frequency region with a negative real part of μ. In the combined structure, at frequencies where both ε and μ have negative real parts, the real part of the refractive index is also unambiguously negative. The authors demonstrate that the traditional procedure of obtaining material parameters from transmission/reflection data can be successfully applied to metamaterials, addressing potential ambiguities in defining the first surface of the measured sample and the sample length. They also discuss the implications of chiral behavior in the data, suggesting that further improvements to the technique are needed for a full characterization of metamaterials.The paper by D. R. Smith and S. Schultz, along with P. Markos and C. M. Soukoulis, analyzes the reflection and transmission coefficients of electromagnetic metamaterials to determine their effective permittivity (ε) and permeability (μ). The authors use transfer matrix simulations on finite lengths of metamaterials, including periodic arrangements of wires, split ring resonators (SRRs), and a combination of both. They find that the recovered frequency-dependent ε and μ are consistent with analytic expressions predicted by effective medium arguments. Notably, a wire medium exhibits a frequency region with a negative real part of ε, and SRRs produce a frequency region with a negative real part of μ. In the combined structure, at frequencies where both ε and μ have negative real parts, the real part of the refractive index is also unambiguously negative. The authors demonstrate that the traditional procedure of obtaining material parameters from transmission/reflection data can be successfully applied to metamaterials, addressing potential ambiguities in defining the first surface of the measured sample and the sample length. They also discuss the implications of chiral behavior in the data, suggesting that further improvements to the technique are needed for a full characterization of metamaterials.