We report on a 100 \% transmission in one- and two-dimensional plasmonic metamaterials in the terahertz frequency range. At a specific frequency at which the plasmonic meta material is perfectly impedance matched to vacuum, the metal material becomes essentially invisible. In the case of a two-dimensional array of square holes, perfect impedance matching occurs when the effective dielectric constant becomes --2 for samples with hole coverages larger than 0.19. In a one-dimensional array of slits, transmission becomes 100 \% when a symmetric eigenmode is excited and therefore the system becomes effectively impedance-matched.