This chapter assesses what the diseases of poverty are. Infectious diseases of poverty are widespread in low-income countries. As a result, under-five child mortality is often higher and life expectancy lower in these countries, compared with middle- and high-income countries. There are long standing disagreements about the most effective way to combat diseases of poverty in low-income countries. In particular, there is an enduring debate about whether selective biomedical interventions (such as deworming for neglected tropical diseases) should be the main focus of public health programmes or whether the most effective way forward is to develop a comprehensive approach which strengthens all levels of the health system, including primary health care units, while simultaneously improving access to clean water and sanitation facilities. Many populations no longer live in abject poverty, and the causes of death and disability have changed from infectious diseases to non communicable diseases. These changes are often described as an epidemiological transition. However, it has also been noted that a transitional period frequently occurs in which populations experience the 'double burden' of long standing infectious diseases alongside chronic, non communicable diseases.