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This article deals with the many long time series databases that have been developed over the last two decades. These series cover a span of more than 100 years and allow us to compare the situation of current developed countries at a time when they were developing countries with that of present-day developing countries. Longitudinal econometric analyses are now feasible instead of cross-sectional analyses of recent data concerning developing and already developed countries. The field of development economics is thus significantly enlarged. This opens up new perspectives and allows us to test models and to understand the role of exogenous factors such as technology or institutions, which contribute to assessing the validity of models in two different periods. This article offers examples that show how useful such long time series, such as education data, can be to explaining variations in, for example, fertility rates.