Abstract: Electromagnetically induced transparency (EIT) is essential in coherent atomic physics, which has been extensively studied in the last two decades. The important properties of EIT are its steep normal linear dispersion and greatly enhanced nonlinearity with a negligible absorption near atomic resonance. When EIT medium is placed inside an optical cavity, many interesting effects can occur. In this talk, I will discuss several experiments in EIT-related atom-cavity systems. Particular attention will be given to nonlinear optical cavity transmission spectra and optical multistate switching with Doppler-broadened multilevel atoms inside an optical ring cavity. In addition, parity-time symmetry and optically induced lattice in EIT-related medium will be briefly discussed.