Human capital is one of the pillars of economic growth and development. However, the economic analysis of human capital presents a large number of difficulties, especially since the mechanisms that allow the accumulation of this type of capital, skills and knowledge, are intrinsic to human nature and, in general, are difficult to determine. But, in addition, the formation of human capital results from formal and informal learning and occurs throughout life, so its measurement is a challenge. Trying to overcome the drawbacks that the analysis of human capital poses, some researchers have proposed the differentiation of human capital into types or levels in order to see its differential effects on development, arguing that only useful knowledge, that is, that which it can be applied to the transformation of reality is the one that is really involved in the processes of economic growth. In the case of Spain, it has been argued that the lack of human capital severely limited economic growth and the subsequent modernization of the country. Although the central explanation lies in the low social demand for education and inaction on the part of the State, there are few studies that have delved into the issue from an aggregate perspective. However, except for some broad approaches to secondary education, there are no national studies on the origins and early stages of secondary education in Spain that contain a systematic statement of the number of students, the number of public and private centers that They imparted these lessons and, ultimately, an analysis of the potential effects of extending the stage on the trajectories of economic development and on regional inequality in the medium and long term. This doctoral thesis aims to provide an answer to the question: what did not work in Spain so that secondary education did not become general as it did in other countries, where it also played an important role in the processes of economic development? The obvious answer to this question has to do with the low literacy rates that Spain had in the 19th century and, therefore, the reduced capacity of the system to generate candidate students to access and successfully pass further educational stages. However, we believe that the social and economic aspects intrinsic to the question at hand are sufficiently varied and complex to suggest nuanced answers that go beyond the literacy argument. The different chapters of this thesis analyze various aspects of secondary education in Spain in the second half of the 19th century and the first third of the 20th century, both from the point of view of supply and from that of demand, in order to shed light about the limitations that the educational stage had to face during the first years of its development.Human capital is one of the pillars of economic growth and development. However, the economic analysis of human capital presents a large number of difficulties, especially since the mechanisms that allow the accumulation of this type of capital, skills and knowledge, are intrinsic to human nature and, in general, are difficult to determine. But, in addition, the formation of human capital results from formal and informal learning and occurs throughout life, so its measurement is a challenge. Trying to overcome the drawbacks that the analysis of human capital poses, some researchers have proposed the differentiation of human capital into types or levels in order to see its differential effects on development, arguing that only useful knowledge, that is, that which it can be applied to the transformation of reality is the one that is really involved in the processes of economic growth. In the case of Spain, it has been argued that the lack of human capital severely limited economic growth and the subsequent modernization of the country. Although the central explanation lies in the low social demand for education and inaction on the part of the State, there are few studies that have delved into the issue from an aggregate perspective. However, except for some broad approaches to secondary education, there are no national studies on the origins and early stages of secondary education in Spain that contain a systematic statement of the number of students, the number of public and private centers that They imparted these lessons and, ultimately, an analysis of the potential effects of extending the stage on the trajectories of economic development and on regional inequality in the medium and long term. This doctoral thesis aims to provide an answer to the question: what did not work in Spain so that secondary education did not become general as it did in other countries, where it also played an important role in the processes of economic development? The obvious answer to this question has to do with the low literacy rates that Spain had in the 19th century and, therefore, the reduced capacity of the system to generate candidate students to access and successfully pass further educational stages. However, we believe that the social and economic aspects intrinsic to the question at hand are sufficiently varied and complex to suggest nuanced answers that go beyond the literacy argument. The different chapters of this thesis analyze various aspects of secondary education in Spain in the second half of the 19th century and the first third of the 20th century, both from the point of view of supply and from that of demand, in order to shed light about the limitations that the educational stage had to face during the first years of its development.