PIP: Mexico's demographic transition was much later and more rapid than the classic transitions of European populations. A careful study of available data sources, especially fertility surveys, allows a detailed understanding of the reproductive process in Mexico, including the nuptiality patterns that influence fertility as well as changes in general and marital fertility. This work assesses the data sources and methods utilized to analyze Mexican fertility in the past, reviews fertility trends before the onset of the transition from about 1940-70, and describes the new reproductive patterns observed since about 1976. Fertility information from the decinnial censuses is not very adequate for measuring fertility levels or trends. Possible estimates based on census information are few and widely scattered in time, and omissions, underregistration, and faulty declarations are common because of the retrospective nature of the census. Census information is highly aggregated so that significant intermediate variables are not easy to assess. But census data are still the only source allowing estimations of fertility levels by areas of residence, administrative entities, and their social and economic characteristics. Mexico's vital statistics are of poor quality, with underregistration and late registration common. Evaluations of the Mexican Fertility Survey (EMF) of 1976-77 indicate that it provides more reliable estimates than the vital statistics or the census. The EMF and the National Demographic Survey (END) of 1982 indicate that the total fertility rate fell by 30.3% between 1974-80, from 6.27 to 4.37. Both the EMF and the END were national level surveys which collected complete fertility histories thus providing longitudinal information on the marital and fertility histories of women aged 15-49. This work uses primarily data from the EMF and END to analyze the period of fertility increase between 1940-60, the period of highest fertility between 1950-70, and the transition from a natural fertility regime to one of fertility control after 1970. The analysis contains 2 major parts, 1 presenting a study of general fertility including age specific rates for generations and periods, final family size of women terminating their childbearing, and the calendar of fertility for women still of fertile age. The 2nd major section focuses on marital fertility using the same indicators but taking into account the relationships between nuptiality and fertility. Comparisons are included between fertility levels based on the major surveys and those implied by the census and vital statistics data.^ieng