Citizens-creditors. Municipalities and Financial Markets in Spain during the First Third of the Twentieth Century - Modernisation is a complex process that involves several aspects and different scientific disciplines. Social and institutional modernisation, particularly that of municipalities, which took place between the late 19th century and the early of 20ths, involved significant changes over wide areas. These areas range from political and institutional internal reorganisations, to the application of new mass services and infrastructural networksystems, as well as basic social and organizational changes. In the following pages we study the measures taken by a wide segment of Spanish municipalities as part of their internal modernization processes shortly after the critical events of the late 19th century. To do this, they made use of the emerging financial markets by raising additional funds, and more precisely, by regularly emitting public debt. In this article we will show how the use of formal market debt involved certain preconditions to be carried out by the emitting organisms, and the underlying advantages to be had. Financial leverage obtained by using market facilities became a powerful instrument for local change in Spain that where up to that time no change had been possible. But the financial crisis of the 1930s and the later Civil War set a downward course in the modernisation process and even in the use of financial markets by Spanish municipalities.