This article aims to analyse the reasons for the intensive use of child labour in the 19th century and its subsequent decline in the first third of the 20th century in the context of an economy with a highly flexible labour supply like that of Catalonia. During the second half of the 19th century,factors relating to family economies, such as numerous families and low wages for adults, along with the technologies of the time that required manual labour resources, would appear to explain the intensive use of child labour to the detriment of schooling. The technological changes that occurred during the first third of the 20th century, the demographic transition and adult wage increase (for both men and women) explain the schooling of children up to the age of 15 and the consequent practical abolition of child labour in that new era of economic modernisation.