In 2004, Spain's Banco Santander purchased Britain's Abbey National Bank in a deal valued at fifteen billion dollars--an acquisition that made Santander one of the ten largest financial institutions in the world. Here, Mauro Guill n and Adrian Tschoegl tackle the question of how this once-sleepy, family-run provincial bank in a developing economy transformed itself into a financial-services group with more than sixty-six million customers on three...