Electromagnetic ion cyclotron (EMIC) waves are transverse plasma waves generated in the inner magnetosphere by ring current ions with perpendicular temperature anisotropy and are typically registered on the ground in the Pc1-2 frequency range. This chapter focuses on the characteristics of EMIC waves that may be important for understanding their impacts on ring current and radiation belt dynamics. It examines wave characteristics in the source region, and considers the effect of plasma composition on the wave spectrum and propagation to the ground. The chapter also outlines the potential importance of the plasmapause for wave growth and ducting. It shows that EMIC wave activity is often observed to be generated in radially narrow regions, just inside the plasmapause. Analysis of satellite plasmapause crossings demonstrates that EMIC waves observed in the vicinity of the plasmapause have relatively low occurrence rates and are observed in lower than 10% of cases.