The paper by D. R. Smith and D. Schurig explores the behavior of electromagnetic wave propagation in materials with indefinite permittivity and permeability tensors, where not all principal elements of these tensors have the same sign. This study reveals a wide range of effects, including negative refraction, near-field focusing, and high impedance surface reflection. The authors demonstrate that a bi-layer of such materials can transfer field distributions, including near-fields, without requiring internal exponentially growing waves. They also discuss the broader context of structured media, such as photonic band gap materials and metamaterials, which have expanded the range of available electromagnetic material properties. The paper highlights the unique properties of anisotropic media with indefinite $\varepsilon$ and $\mu$ tensors, which can support hyperbolic dispersion relations and exhibit unusual wave propagation behaviors. The authors provide a detailed analysis of wave refraction at interfaces and the design of compensated bilayers to enhance transparency and reduce impedance mismatch. They also explore the high reflection properties of uncompensated semi-infinite indefinite media, which can be used for enhanced antenna efficiency and shielding. The study concludes with the potential for practical applications and the anticipation of further research in this area.The paper by D. R. Smith and D. Schurig explores the behavior of electromagnetic wave propagation in materials with indefinite permittivity and permeability tensors, where not all principal elements of these tensors have the same sign. This study reveals a wide range of effects, including negative refraction, near-field focusing, and high impedance surface reflection. The authors demonstrate that a bi-layer of such materials can transfer field distributions, including near-fields, without requiring internal exponentially growing waves. They also discuss the broader context of structured media, such as photonic band gap materials and metamaterials, which have expanded the range of available electromagnetic material properties. The paper highlights the unique properties of anisotropic media with indefinite $\varepsilon$ and $\mu$ tensors, which can support hyperbolic dispersion relations and exhibit unusual wave propagation behaviors. The authors provide a detailed analysis of wave refraction at interfaces and the design of compensated bilayers to enhance transparency and reduce impedance mismatch. They also explore the high reflection properties of uncompensated semi-infinite indefinite media, which can be used for enhanced antenna efficiency and shielding. The study concludes with the potential for practical applications and the anticipation of further research in this area.