Can different therapists, motivated by different theories and working in different settings, find a single measure - a 'core' - for monitoring their work? This paper introduces the Clinical Outcomes in Routine Evaluation (CORE) outcome measure, its philosophy, practice and utility. The measure is brief and acceptable to clients and therapists. It covers well being, problems/symptoms, life functioning and risk to self and others. It is easy to score by hand and is computer scannable. It measures individual differences on entry into therapy and change. Analyses of over 2,000 responses show good reliability and convergent validity against longer and less general measures; small gender effects; large clinical/non-clinical differences and good sensitivity to change. Referential clinical and non-clinical distributions and reliability enable calculation of clinically significant and reliable change criteria. Two short forms are available for frequent use within therapy and a centrally supported system will scan questionnaires and provide standardised reports and raw data.