This chapter highlights what psychologists have discovered about the role of police investigative processes in causing miscarriages and how their research findings have helped to kick-start change agendas relating to those processes, which have in due course involved legislative, policy and practice reforms. It outlines the contribution of psychological research to researcher's understanding of the causes that are connected to police investigative processes, focusing specifically upon issues surrounding: the investigative process generally, including investigative philosophy, strategy and tactics; interview practices involving suspects, victims and witnesses; and identification procedures. The chapter discusses the stressing that many opportunities remain for psychologists to conduct research in this area, so as to continue to work towards reducing miscarriages of justice. Official recognition of the causes of miscarriages of justice had in part driven such reforms. Many miscarriages of justice seem to highlight that investigators in the cases operated on preconceived notions of guilt.