This work explains how the second half of the 19th century was the key stage in the modernisation of the city of Barcelona, from the demolition of the walls to the aggregations of the towns in the plan. This process is indissolubly linked to Ildefons Cerdà, often seen as a brilliant figure, but isolated and even confronted with the Barcelona environment. However, if this had really been the case, his work would not have had such a coherent continuity as that which was assured, in the following period, by the very different personality of Mayor Rius i Taulet. The idea behind this book is the close link between Cerdà and Barcelona in a perspective of the formation of the metropolis. The work shows the complex web underlying Cerdà's conception of the Eixample, the energy of a society shaken by the industrial revolution and shaped by the liberal regime and, naturally, the difficulties that had to be overcome.