Criminal justice systems throughout the world have long considered jury trials to be an essential feature of a fair and just due process. Grounded in the premise that involvement of lay decision-makers provides an important safeguard against unfair laws and prosecutions, the use of jurors within legal disputes is often attributed to defending against overbearing state control and thereby considered to represent the cornerstone of a demographic society. Today, jury trials continue to be both valued and criticised for including ordinary citizens with no formal legal training in the most serious trial proceedings, where they are ultimately required to determine guilt (or lack thereof). Although attracting considerable scepticism, especially from countries that rely on legally trained judges to make decision at court, jury trials continue to be used in around 25% of countries around the world. Despite this, the scientific study of this model of justice remains notoriously difficult to test, with legal restrictions often preventing researchers from gaining access to the jury room and jurors themselves. For example, in England and Wales, various legislation prohibits jurors from disclosing any aspect of their deliberations, a restriction which many researchers believe has hindered our understanding of how fair juries truly are. Whilst social scientists have developed a number of alternative ways in which jury decision-making can be assessed, the need for more ecologically valid empirical research remains apparent. In this chapter, you will be introduced more thoroughly to the ideology underpinning jury trials and the common psychological research methods used to investigate the reliability of jury decision-making. You will also be introduced to key psychological theories that attempt to explain exactly how jurors arrive at their verdict decisions at trial.