This chapter is a comprehensive reference manual providing information on the Overt Aggression Scale – Modified for Neurorehabilitation (OAS-MNR), which is completed following direct observation of an aggressive act. The OAS-MNR is a more complex instrument than the standard rating scale, but this is in part a function of producing an instrument that measures contextual factors surrounding the aggression act, in addition to the frequency and type of aggression. This feature makes the OAS-MNR a superior instrument. Consequently, the information yielded is sufficiently detailed to provide the basis for an individually tailored therapeutic programme. The OAS-MNR documents the antecedents to the aggressive behaviour, along with the interventions that were implemented. A brief history of the instrument's development is provided, along with a description of item content and administration/scoring procedures. Psychometric properties are reviewed and a critical commentary is provided. Key references are cited and in most cases the actual scale is included, giving the reader easy access to the instrument.