As you learned in Chapter 1, one of the main uses of epidemiology is to identify the causes of disease and this is of fundamental importance in all areas of public health – if we can work out what is causing ill-health then we can work to prevent it. In Chapter 2 we looked at the ways in which we can measure the occurrence of disease and touched on some ways in which we can compare different populations. But while measuring the occurrence of a disease in a population can tell us about the health of that population, it does not directly shed much light on the underlying causes of the disease. To identify the aspects of people or their environment (exposures) that might lead to the onset of disease, we need to compare disease occurrence in groups with and without the exposures of interest. In Chapter 4 we looked at some of the study designs that we can use to do this; now we will look more closely at the measures we use to quantify the associations between ‘exposures’, or potential causes of disease, and the disease itself.