It is sometimes suggested that innovation is inherently impossible to quantify and to measure. This chapter argues that while this is true for some aspects of innovation, its overall characteristics do not preclude measurement of key dimensions of processes and outputs. An important development has been the emergence of new indicators of innovation inputs and outputs, including economy-wide measures that have some degree of international comparability. Following sections discuss first some broad issues in the construction and use of science, technology, and innovation (STI) indicators, then turn (briefly) to the strengths and weaknesses of current indicators, particularly R&D and patents. Final sections cover recent initiatives focusing on the conceptualisation, collection, and analysis of direct measures of innovation.