The Latino Big Bang in California presents a Spanish transcription and English translation of a diary written by Forty-Niner Justo Veytia, a Mexican immigrant seeking riches during California’s Gold Rush. Veytia’s diary offers insights into the dilemmas and choices of an adventurous and ambitious young mexicano and provides a detailed glimpse into the life of Latinos who participated in this tumultuous moment in California history. In doing so, Veytia’s diary demonstrates that the US-Mexico War together with the Gold Rush constituted a Latino “big bang” in California that attracted large swaths of fortune seekers from across the Spanish-speaking world throughout the latter half of the nineteenth century. Combining archival research with quantitative methods to extrapolate demographic information about the persistent presence of Latino communities in California from the mid-nineteenth century to today, The Latino Big Bang in California shows how Latino migration and labor forever changed the course of California history.