In 3d-chiral media, circularly polarized electromagnetic waves of opposite handedness can propagate with different speed. This phenomenon is known as circular birefringence and described by different real parts of refractive indices for left- and right-handed circularly polarized waves. As a consequence, left- and right-handed circularly polarized waves accumulate different amounts of phase upon propagation through a chiral medium. This phase difference causes rotation of the polarization state of linearly polarized waves, which may be thought of as superposition of left- and right-handed circularly polarized waves. Circular birefringence can yield a negative index of refraction for waves of one handedness when the effect is sufficiently large.