The power thyristor is the most important semicoductor device used in the control of electric power. An explanation of thyristor ratings and rating presentation is required for the complete understanding and successful application of these devices. This paper not only explains the meaning of thyristor temperature, voltage, current, and gate ratings but also presents insights into how these ratings are developed. Both the semiconductor controlled rectifier (SCR), which is properly called a reverse blocking triode thyristor, and the bidirectional thyristor rating methods are discussed. SCR rating are further divided into those applying to phase control applications at power frequencies and those applying to high repetition rate inverter and pulse current appications.