Metal-insulator-metal (MIM) structures have been the subject of great interest as nanoscale plasmonic waveguides. The modeling and measurement of the loss in these waveguides is one of the critical issues in realizing the plasmon-based nanocircuitry. Due to the subwavelength size of the structure, the light injection and the measurement of the loss in MIM structures typically require tapered fibers or waveguides, as well as multiple waveguide structures with various length scales 8 9 or scanning near-field optical microscopy. The transverse transmission/reflection (TTR) method is presented for determining the loss of plasmonic modes in MIM waveguides. The approach is based on determining the width of the reflection angular spectrum in the attenuated total reflection configuration. Owing to its transverse character, the TTR method potentially provides a more straightforward and simpler way to determine the loss of plasmonic modes in MIM structures.