Measuring association between variables is very relevant for investigating causality, which is, in turn, the sine qua non of scientific research. However, an association between two variables does not necessarily imply a causal relationship, and the research design of a study aimed at investigating an association needs to be carefully considered in order for the study to obtain valid information. Knowledge of measures of association and the related ideas of correlation, regression, and causality are cornerstone concepts in research design. This entry is directed at researchers disposed to approach these concepts in a conceptual way.