Stigma against mental illness has devastating consequences for individuals with mental illness and their families. Empirical findings and qualitative evidence indicate that stigma against mental illness remains rampant in many nations and cultures, constituting a significant barrier to successful treatment, reducing key life opportunities, and predicting poor outcomes over and above the effects of mental illness per se. In this article we define stigma, examine relevant theoretical perspectives, summarise evidence regarding the pervasive negative impact of stigma on individuals with mental illness, and discuss underlying mechanisms. We focus in particular on assessment issues, highlighting the need for transcending explicit attitudinal measures of stigma, which are susceptible to social desirability concerns and are likely to underestimate true levels of stigma, to include unconscious/implicit indicators and direct behavioural appraisals. A primary goal is to facilitate means of accurately measuring stigma against mental illness as an important step toward reducing its pernicious effects.