A review is given of Ohmic and Schottky contacts to n- and p-type ZnO. It is relatively straightforward to form high-quality Ohmic contacts to n-type ZnO, with specific contacts resistivity in the range 10(-6) Omega cm(2) even for unnannealed contacts on strongly n-type layers, while recent work has also shown good results (10(-5)-10(-6) Omega cm(2)) for Au or Ni/Au annealed at 300-600 degrees C. Schottky contacts to both nand p-type ZnO are much lower than expected from the metal work function and the electron affinity of ZnO, suggesting that surface states are important in determining the effective barrier height. The Schottky contacts also show poor thermal stability. For transparent transistors based on ZnO, this suggests that oxide gates are more suitable than metal gates. (c) 2005 Elsevier B.V. All rights reserved.