The wealth of studies carried out on infant mortality testifies to its continuing fascination for the modern researcher. This chapter discusses the development of infant mortality as a problem of national importance at the end of the nineteenth century. It explains persistently high levels of infant mortality, Sir George Newman focused on conditions within the towns and cities and on the timing of death within the first year of life; both issues which continue to be pursued by contemporary analysts. The chapter focuses on the causes and effects of infant diarrhoea, a disease to which Newman devoted much space in his own account. It uses some especially revealing sources to unravel the effects of parental employment and residential location on infant mortality during the mid-nineteenth century. This introduction presents an overview of the key concepts discussed in this book.