Quantitative information about immigration of Latinos into Boulder County is presented in this resource. It draws upon data from the U.S. Censuses, 1900-1940, and the School Census that each district in Colorado had to submit annually. The state provided a booklet into which information was to be entered about each student, including where the child was born and in some years where the parents were born. For Longmont, the School Census books have been sampled once per decade, 1905-1964. Lafayette’s and Boulder’s School Censuses are less complete, covering only some years between 1915 and 1955. These sources provide information about initial migration from Mexico, followed by migration from New Mexico and southern Colorado; and they show Mexican-born men marrying U.S.-born women. They also document that as early as 1930, 69% of the local Latino population had been born in this country: they and their children were by definition U.S. citizens. By 1940, that figure had reached 84%. The text and spreadsheets included here offer an ideal way to integrate Latino material into work in mathematics, using spreadsheets, and mapping.