A Lexis diagram in its standard form is a coordinate system with calendar time as abscissa and age as ordinate, representing individual lives by line segments with slope 1, starting at (date of birth, 0) and ending at (date of death, age at death). Lexis diagrams are important tools in demography and epidemiology (age–period–cohort models). This article outlines the history of the diagram from the age of the German mathematician and economist W. Lexis and mentions generalizations and applications up to the present day