The Study reviews existing national and EU rules on the definition, identification and procedural rights of vulnerable adults who are suspects or accused persons in criminal proceedings. It identifies the main problems and explores possible solutions to address these at EU level. Member States' legislation reflect only partially the provisions of the 2013 Commission Recommendation on procedural safeguards for vulnerable persons suspected or accused in criminal proceedings. There is no common definition of vulnerable adults and the definitions used for specific procedural safeguards diverge across Member States. The identification of vulnerabilities from the early stages of criminal proceedings is not ensured systematically and adequately. Vulnerable adults are often not effectively represented and assisted. Procedural safeguards do not apply systematically nor at the same level for each stage of the procedure. These divergences impact upon cross-border judicial cooperation and mutual trust. Among the different policy options which could address these problems, the most promising would be a combination of the adoption of a specific legislative instrument, i.e. a directive, to introduce strengthened procedural safeguards for vulnerable adults, hence completing the current set of directives which regulate procedural rights with specific safeguards for vulnerable adults, and of non-legislative measures.