This article presents the results of the family reconstitution carried out in three villages in the Spanish province of Navarre, covering a period from the late seventeenth century to 1994. This set of data from over 300 years comprises one of the longest family reconstitution projects in Spain, and means that the present study is based on a major database consisting of over 110,000 entries which can be used to explore various aspects of the demography. In this study I present the evolution of several indicators of fertility, all of which are aspects that can help us to reflect on a phenomenon that has long been known, but which is still not fully understood: the demographic transition. The microfocus of family reconstitution has enabled us to question numerous assumptions about the time when the fall in the birth rate began, the irrational or unconscious nature of reproduction in the pretransitional phase and the strategies employed by couples to have smaller families.