Many decades after the first descriptions of hay fever in the 19th century, we still struggle with the formation of a cogent conception embracing all the many fragments of insight into the various aspects of allergic and asthmatic diseases. Although in daily practice we can grasp and allocate most of the clinical manifestations, in research the fundamental structures are only in part recognized. We therefore presume that the underlying mechanisms are complex and multifaceted. We have explored in countless directions while branching into ever more detailed analyses of phenomena, all of which are associated with the conditions of interest. But how can we rearrange these pieces to advance toward a broader understanding of asthma and allergies? Epidemiology as a scientific tool to study the occurrence, the natural course, and the determinants of a condition in various sociocultural environments might contribute to the edifice of such a scaffold. This review does not intend to present a comprehensive digest of the relevant literature: the reader can refer to other "state of the art" reviews. The intention is to discuss some conceptual cornerstones and to illustrate them with a few examples.