This paper describes that Ge plays an enabler to integrate active photonic devices on a Si platform. In spite of the large lattice mismatch of similar to 4% between Ge and Si, high-quality Ge layers can be epitaxially grown on Si by ultrahigh-vacuum chemical vapor deposition. Applications of the Ge layers to near-infrared active photonic devices, i.e., photodiodes, optical modulators and light emitters, are described. Several issues on the device physics as well as the integration with Si electronics are discussed. (C) 2009 Elsevier B.V. All rights reserved.