A lay adjudication system that has been the subject of international and domestic reforms democratising criminal justice procedures often reveals a challenging balance amongst the intertwined principles of criminal procedures. In addition, the various stages of lay adjudication procedures, such as its introduction, practice, development, and abolition, have been influenced by political and societal force. Lay adjudicator participation may be an important mechanism for introducing and sustaining the concept of democracy in criminal trial procedures. Whilst the establishment of well-balanced links between international fundamental human rights principles, efficiency and popular justice are significant features in judicial reforms internationally and domestically, the introduction of a lay adjudication procedure can be an appealing addition to judicial reform. A fundamental concern is whether it represents a successful mechanism for democratising criminal procedures. This thesis addresses the issue by examining the 'Saiban-in no Sankasuru Keijisaiban ni kansuru Horitsu', promulgated in 2009, as a result of the 1999 judicial reform in Japan. It does this by firstly setting out evaluative criteria developed through an examination of theoretical perspectives of lay adjudication. It then applies these criteria using quantitative data derived from the Japanese Supreme Court and qualitative data from interviews with former citizen judges and legal professionals who have experience of citizen judge trials. It argues that the introduction and practice of the citizen judge system has been successful. Both procedural and practice tests of the citizen judge system have shown the extent to which citizen judge participation has been accepted and has achieved its targets. The representative and engaging format of the citizen judge system has led to satisfaction and confidence in their duties as citizen judges. However, powerful controls by legal professionals have remained in place throughout the four stages of the citizen judge procedures - the pre-trial arrangement conference process, the selection process of citizen judges, the decision-making process, and the post-trial phrase. Moreover, the controls have supported citizen judges’ participation but, at the same time, they could be a direct and indirect impediment to the democratic functions of citizen judges.