The essence of a well performing ring laser gyro is the complete reciprocity of the traveling wave oscillator. In addition to this, the scale factor needs to be sufficiently large and above all extremely stable. In an active ring laser gyroscope, the gain medium inside the resonator causes a modification to the scale factor of the instrument, which is important to quantify correctly in order to relate the observed optical beat note properly to the experienced rate of rotation of the sensor. For highly accurate rotation sensing, these effects have to be known with extremely high accuracy. This chapter introduces the important helium–neon-based ring laser gyroscope design features.