Werner Israel has conducted important work in general relativity, particularly on black holes and relativistic thermodynamics. He proved the unexpected theorem that a non-rotating black hole must be spherically symmetric. His theorem initiated a chain of results which led to the complete description of black hole equilibrium states by known solutions. This opened a new era in black hole theory by making it possible to perform detailed calculations. He has also shown how to treat thin shells of matter in general relativity and has developed a theory of transport coefficients in relativistic thermodynamics which does not lead to propagation faster than light.