In this chapter we focus on the measurement of economic agglomeration in the context of the clustering of regional economic activity. We first discuss various agglomeration measures that have been proposed in the literature, to provide alternative methodologies for the direct measurement of agglomeration. We then briefly review the estimation of the determinants or sources of agglomeration, and the resulting agglomeration economies or productivity effects of agglomeration, both of which involve methods of indirect measurement. Also, causality of productivity and firm location are discussed. In conclusion we highlight some topics that it will be important to address in future studies of agglomeration economies.