The prevalence of offenders with learning difficulties and learning disabilities is not agreed upon. What is clear, however, is that, regardless of actual numbers, many offenders have learning difficulties that reduce their ability to cope within the criminal justice system, for example, not understanding fully what is happening to them in court or being unable to access various aspects of the prison regime, including some offending behaviour programmes. Offenders with learning difficulties are not routinely identified and, as a result, often do not receive the support they need. "No One Knows" is a UK wide programme led by the Prison Reform Trust that aims to effect change by exploring and publicizing the experiences of people with learning difficulties who come into contact with the criminal justice system. The article highlights the aims of "No One Knows" and describes what, for the purpose of the programme, we mean by "learning difficulties and learning disabilities". Problems in identifying precise numbers of offenders with learning difficulties and learning disabilities are discussed and attention drawn to recent research on prevalence. The context and some of the challenges of "prison life" are identified and a number of early research findings from "No One Knows" are presented.