The population of the world is getting older. In 2010, worldwide, there were about 524 million people over the age of 65; by 2050, over 1.5 billion people will be in this age group. This shift in population will not affect only developed countries, however—much of this increase in the elderly population will occur in low- and middle-income countries. As populations age, low-income countries will need to invest in health care for older adults and in disease prevention programs to prevent or delay the onset of non-communicable diseases (such as heart disease, stroke, and cancer). Past research on population-level health in the developing world has been widely hindered by a lack of high-quality longitudinal data. My dissertation uses recently-collected longitudinal data to gain insight into overall trends in health in low- and middle-income contexts. My first chapter uses a multi-state life table approach to investigate the overall level of health and functional ability (the ability to carry out tasks of daily life) among the rural population in Malawi. I find that this population experiences a substantial burden of disability in later life, and that these high levels of disability greatly limit work efforts among older individuals. In my second chapter, I conduct a cross-national comparison of health and disability-free life expectancy using data from recent longitudinal surveys in Costa Rica, Mexico, Puerto Rico, and the US. I find that current disability-free life expectancy at age 65 is comparable across these populations, though future trends are uncertain. My third chapter investigates how Malawi’s 2008 rollout of Anti-Retroviral Therapy (ART) to rural clinics affected overall population health and mortality. I find that the introduction of ART led to substantial declines in mortality and an increase in adult life expectancy, and that population morbidity also decreased after the introduction of ART.