Chapter 4 tackles the issue of measuring harbors. It begins with a brief survey of maritime history and port construction, which informs the measurement approach. The focus is on natural harbors: those locations that are naturally suited to protect ships near shore. These are identified using a variety of historical sources. However, since port construction is not limited to natural harbors, a predictive model of natural harbors is also developed, based on coastal geography, that can be used to measure the distance between any grid-cell and the nearest natural harbor. The remainder of the chapter is devoted to a discussion on issues of spatial autocorrelation and the trade-offs between the use of grid-cells or countries as the unit of analysis.