Purpose This paper is the first in a series that examines methods for improving the validity and utility of randomized clinical trials (RCTs) in addictions treatment research. The specific foci of this article are treatment implementation and research design. Scope We begin by considering the conditions under which the RCT provides an appropriate design choice. Sections that follow discuss methodological issues with respect to RCT structure and collaborative arrangements; treatment specification, delivery and cost; experimental design; and randomization/blinding procedures. We emphasize the importance of advance planning; treatment integrity and discriminability; treatment standardization; staff training and supervision; client compliance; maintenance of between-group equivalence across study conditions; and inclusion of appropriate comparison groups in study designs. Conclusions Investigators are encouraged to maximize the internal validity of RCTs, but also to consider methods for enhancing external validity. The utility of addictions RCTs for advancing theory and improving clinical practice can be enhanced by investigating underlying mechanisms of action.