This supplementary material provides additional data and methods to support the findings of the article "Accelerated modern human-induced species losses: Entering the sixth mass extinction." The authors, Gerardo Ceballos, Paul R. Ehrlich, Anthony D. Barnosky, Andrés García, Robert M. Pringle, and Todd M. Palmer, published their research in *Sci. Adv.* on June 19, 2015. The supplementary materials include: 1. **Data on Contemporary Extinctions (1500 to Present)**: The authors obtained a list of vertebrate species considered Extinct (EX), Extinct in the Wild (EW), and Possibly Extinct (PE) since 1500 from the International Union for Conservation of Nature (IUCN). They accessed the IUCN webpage on December 1, 2014, and verified the status of "possible extinct" species provided by Dr. Bruce Young from Natureserve. 2. **Background Extinction Rates**: The background extinction rate for mammals was determined by Barnosky et al., based on an analysis of mammal fossils, subfossils, and modern extinctions. The new rate is estimated to be 2 extinctions per million species-years (2 E/MSY), which is significantly higher than previous estimates of 0.1 to 1 E/MSY. 3. **Comparing Recent and Background Extinction Rates**: The authors used two estimates of modern extinction rates: a very conservative rate (including only EX species) and a conservative rate (including EX, EW, and PE species). Table S2 summarizes the calculations for these rates, showing that highly conservative rates are calculated using only EX species, while conservative rates include EX, EW, and PE species. The background extinction rate of 2 E/MSY is equivalent to two extinctions per 10,000 species per 100 years. These supplementary materials provide detailed data and methods to support the analysis of modern extinction rates and their comparison with historical rates.This supplementary material provides additional data and methods to support the findings of the article "Accelerated modern human-induced species losses: Entering the sixth mass extinction." The authors, Gerardo Ceballos, Paul R. Ehrlich, Anthony D. Barnosky, Andrés García, Robert M. Pringle, and Todd M. Palmer, published their research in *Sci. Adv.* on June 19, 2015. The supplementary materials include: 1. **Data on Contemporary Extinctions (1500 to Present)**: The authors obtained a list of vertebrate species considered Extinct (EX), Extinct in the Wild (EW), and Possibly Extinct (PE) since 1500 from the International Union for Conservation of Nature (IUCN). They accessed the IUCN webpage on December 1, 2014, and verified the status of "possible extinct" species provided by Dr. Bruce Young from Natureserve. 2. **Background Extinction Rates**: The background extinction rate for mammals was determined by Barnosky et al., based on an analysis of mammal fossils, subfossils, and modern extinctions. The new rate is estimated to be 2 extinctions per million species-years (2 E/MSY), which is significantly higher than previous estimates of 0.1 to 1 E/MSY. 3. **Comparing Recent and Background Extinction Rates**: The authors used two estimates of modern extinction rates: a very conservative rate (including only EX species) and a conservative rate (including EX, EW, and PE species). Table S2 summarizes the calculations for these rates, showing that highly conservative rates are calculated using only EX species, while conservative rates include EX, EW, and PE species. The background extinction rate of 2 E/MSY is equivalent to two extinctions per 10,000 species per 100 years. These supplementary materials provide detailed data and methods to support the analysis of modern extinction rates and their comparison with historical rates.