This paper discusses London Underground's continuous Customer attitude research program: known as the Customer Satisfaction Index (CSI). It is written in the spirit of describing the genesis of an actionable management information tool: one that now forms the prime business goal of the worlds' oldest and most comprehensive urban rapid transit system. The paper covers: the development of the CSI; the interviewing and sampling technique used; how the information is reported and (using real data) shows how changes in the Underground's service affect Customers' satisfaction. Some simple suggestions for maximising response rates and controlling fieldwork costs are given. The paper also explains how this very large, mature organisation is managed, and how that structure is reflected in the research. The CSI is intended to be read and understood by staff at all levels within the Underground: from Train Operators to the Managing Director. The paper concludes by showing the value of a continuous tracking study such as the CSI when outside factors suddenly affect the organisation. This is illustrated by reference to CSI research carried out after a spate of terrorist attacks on London's transport network.