This talk will provide an overview of the measurement of wider economic impacts in transport appraisal focusing in particular on the role of agglomeration economies. It will describe how agglomeration effects can be estimated and included within a standard Benefit Cost Analysis framework to assess the productivity benefits of investment. Examples from around the world will be used to illustrate the potential significance of agglomeration externalities. The talk will discuss limitation of the existing approach, particularly in relation to econometric modelling, and will suggest future areas for research.