The use of conformal radiotherapy (RT) and the follow-up of patients for radiation liver toxicities has led to a quantitative understanding of partial liver RT tolerance. The most common toxicity is radiation-induced liver disease (RILD), a syndrome of anicteric ascites and hepatomegaly. Elevation of transaminases and reactivation of viral hepatitis have also been reported after liver RT. The Lyman normal tissue complication probability (NTCP) model and a local damage-organ injury NTCP model have been used to describe the partial tolerance of the liver to RT. The liver exhibits a large volume effect and a low threshold volume for RILD. The RT tolerance of the liver is reduced in patients with primary liver cancer versus metastases. Elevated transaminases are more common in the presence of poor liver function and hepatitis B infection. If the effective liver volume irradiated is less than 25%, very high RT doses may be delivered with little risk of liver toxicity. The mean liver doses associated with a 5% risk of classic RILD for primary and metastatic liver cancer are 28 Gy and 32 Gy, respectively, in 2 Gy per fraction.