Background Comprehensive and accurate analyses of causes of death, disaggregated by age, sex, and location, are essential for formulating effective health policies aimed at reducing global mortality. The Global Burden of Diseases, Injuries, and Risk Factors (GBD) 2023 study provides estimates of cause-specific mortality, measured in numbers, rates, and years of life lost (YLLs). The GBD 2023 study aims to enhance our understanding of the relationship between age and cause of death by determining the probability of dying before age 70 (70q0) and the median age at death by cause and sex. This study enables comparisons of the impact of causes of death over time, providing a deeper understanding of how these causes affect the global population. Methods The GBD 2023 report produced estimates for 292 causes of death, disaggregated by age, sex, location, and year, for 204 countries, territories, and 660 subnational locations for each year from 1990 to 2023. We used a modeling tool developed specifically for the Global Burden of Disease report, the Cause-of-Death Ensemble Model (CODEm), to estimate cause-specific mortality rates for most causes. We calculated years of life lost (YLLs) as the product of the number of deaths for each cause, age, sex, location, and year, and the standardized life expectancy at each age. The probability of dying was calculated as the probability of dying from a specific cause at a specific age and for a specific population. Life expectancy at death was calculated by determining the median age for each age group for each death and then averaging all median ages for all cause-attributed deaths. We used the Global Burden of Disease mortality estimates to calculate observed life expectancy at death and to model life expectancy across causes, sexes, years, and locations. Life expectancy reflects the life expectancy at death for individuals within a population, based on global mortality rates and the age structure of the population. By comparison, observed mean age represents the actual mean age at death, influenced by all the unique factors of a specific population in a given location, including its age composition. As part of the modeling process, uncertainty intervals (UIs) were generated using the 2.5th and 97.5th percentiles of a 250-drawing distribution for each metric. Results are reported as counts and age-standardized rates. Methodological improvements to cause-of-death estimates in the Global Burden of Disease 2023 report include a correction for misclassification of deaths from COVID-19, updates to the method used to estimate COVID-19, and updates to the CODEm modeling framework. This analysis utilized 55,761 data sources, including vital registration and verbal autopsy data, as well as data from surveys, censuses, surveillance systems, and cancer registries, among others. For the 2023 Global Burden of Disease Study, 312 new years of cause-of-death data from vital registrations, 3 new years of surveillance data, 51 new years of verbal autopsy data, and 144 new years of other data types were added to those used in previous rounds of the Global Burden of Disease Study. Results The early years of the COVID-19 pandemic led to shifts in the established classifications of the leading causes of global mortality: it was ranked first among age-standardized causes of death at level 3 of the Global Burden of Disease cause classification hierarchy in 2021. By 2023, COVID-19 had fallen to 20th place among the leading global causes, reclassifying the two leading causes to their usual rankings across the time series (i.e., ischemic heart disease and stroke). While ischemic heart disease and stroke remain leading causes of death, progress has been made in reducing global age-standardized mortality rates. Four other leading causes also showed significant declines in their global age-standardized mortality rates during the study period: diarrheal diseases, tuberculosis, stomach cancer, and measles. Other causes of death showed divergent gender patterns, particularly for deaths from conflict and terrorism in some locations. There were significant declines in age-standardized rates of years of life lost (YLL) for neonatal disorders. Despite this, neonatal disorders remained the leading cause of global YLL during the period studied, with the exception of 2021, when COVID-19 temporarily became the leading cause. Compared to 1990, there were significant declines in total YLL for several vaccine-preventable diseases, most notably diphtheria, pertussis, tetanus, and measles. In addition, this study determined median age at death for all-cause and cause-specific mortality and found significant variation by sex and location. Global median age at death from all causes increased from 46.8 years (95% UI 46.6-47.0) in 1990 to 63.4 years (63.1-63.7) in 2023. For males, it increased from 45.4 years (45.1-45.7) to 61.2 years (60.7-61.6), and for females from 48.5 years (48.1-48.8) to 65.9 years (65.5-66.3), from 1990 to 2023. The highest median age at death from all causes in 2023 was found in the high-income region, where females were 80.9 years (80.9-81.0) and males were 74.8 years (74.8-74.9). By comparison, the decline