The primary sources for the Early California Population Project are the baptism, marriage, and burial registers produced by the Franciscans of Spanish and Mexican California. Much like parish priests in Europe, missionaries in California were required to keep records for all Native people affiliated with the missions and for the region's Spanish and Mexican population, all of whom were at least nominally Catholic. Thus, whenever the missionaries in California baptized an individual, they, to the best of their abilities, recorded that individual's birthplace, age, parents, marital status, children, siblings, godparents, Spanish name, and any other information they deemed unique or relevant. They also assigned that individual baptism record a unique number. Similarly, when they married or buried an individual, they assigned that individual’s marriage or burial record a unique number, and in these records they nearly always recorded the individual’s Spanish name, age, marital status, place of baptism, family relations, and, if known, baptism record number. Because the separate baptism, marriage, and burial registers for all of California’s twenty-one missions are largely complete, consistently thorough, and in many ways cross-referenced, records from different missions and registers can be linked and sorted by individual. The California mission registers, therefore, contain the information necessary to reconstruct not only the individual life histories of the tens of thousands of Natives and settlers who lived in Alta California but the divergent population dynamics of these groups.

In nearly all instances, ECPP staff did not work directly with original manuscripts but rather with microfilm copies of the originals. Some of this microfilm is part of the Huntington Library’s microfilm collection, but much of it was borrowed from institutions across the state. The Santa Barbara Mission Archive-Library was particularly generous in sharing its microfilm. The University of Santa Clara provided film for Mission Santa Clara, and the Archive of the Archdiocese of San Francisco permitted the ECPP to use microfilm copies of records from many of the missions of northern California. Finally, the Archival Center of the Archdiocese of Los Angeles provided a copy of various records for Mission San Fernando.

No other region of colonial America that became part of the United States has a database of such an extensive set of vital records. The database encompasses records from all twenty-one of the California missions, in addition to the Los Angeles Plaza Church (1826-1848) and the Santa Barbara Presidio (1782-1848). There are a few notable gaps in the documentary record. All sacramental records from Mission San Luis Rey are missing, but the project used the mission’s padrón (a form of a household census) to reconstruct some of the mission’s population. The burial records for Mission Soledad are lost. There are also major gaps in baptisms at Mission San Gabriel, and at San Diego there is a sixteen-year gap in burials after 1831. With these exceptions there are no major gaps in the missions’ sacramental records. Nevertheless, each mission has its own idiosyncrasies, and these are discussed in various mission memos that can be accessed via hyperlinks on the Tips tab.

Beyond the primary goal of transcribing information from the original records to the database, a secondary goal of the project has been to link together the dispersed baptism, marriage, and burial records of individuals to facilitate data retrieval and the creation of histories of individuals and families. It is in the area of record linkage that the ECPP staff have moved well beyond a simple transcription of the original registers. Complicating the linking of records was the fact that the Franciscans tended to use only first names when they identified Natives in the mission records. (For soldiers and settlers, the missionaries always listed both given and family names.) Fortunately, though, the Franciscans also included in their records many other bits of identifying information that permitted project staff to link burial records to baptism records, marriage records to baptism records, and children’s’ baptism records to their parents’ baptism and marriage records. As of 2022, 91% of 71,360 death records have been linked to the deceased’s baptismal record, 72% of 65,170 baptism records that list information on a Spanish-named mother have been linked to the mother’s baptismal record, 65% of 65,015 baptism records that list information on a Spanish-named father have been linked to the father’s baptismal record (65%), and 90% of 27,985 marriage records have been linked to the bride’s baptism record and 89% have been linked to the groom’s baptism record.

Record links are the result of decisions that may not be apparent to ECPP users. Therefore, we have devoted a separate field to the explanation of how each link was made. We have thirty-nine different ways the link can be established. In addition, in cases where the link was made based on very scant information, we have designated the link with an asterisk. Users who believe they have found an incorrect record linkage in the ECPP database, should notify the project staff at