Despite widespread belief that redlining contributed to disinvestment in cities, there has been little empirical analysis of historical lending patterns. The lack of appropriate data and clear definitions of redlining has contributed to this void. This article reviews definitions and methods that have emerged from research on lending in recent years and considers how they can be applied to research on historical redlining. Address-level mortgage data from Philadelphia from the 1940s are analyzed using spatial regression, “hot spot” analysis, and surface interpolation. Employing multiple definitions of redlining that focus on process and outcome, as well as spatial and statistical relationships in lending, the analyses result in a series of map layers that indicate where redlining may have occurred. In addition to providing some evidence of lending discrimination, this article promotes an explicitly spatial view of redlining that has conceptual and methodological implications for research on contemporary and historical redlining.