A variety of recent methodological advances are allowing addiction treatment researchers to develop effective new treatments and to answer more complex questions regarding the effectiveness of treatments for substance use disorders. These methods, which place a premium on internal validity and have reached their epitome in efficacy studies like Project MATCH, include the use of standardised diagnostic criteria, thorough description of the study sample, standardisation of treatments, training of treatment providers, quality control procedures and manipulation checks. The strengths and limitations of research methods which emphasise internal versus external validity will be discussed, as will the implications of each of these for clinical practice.