Opioid substitution with oral methadone is the most widely implemented treatment for heroin dependence. In Britain, a complex array of specialist and generic health care services has evolved to deliver this treatment modality. Several minority aspects of British substitution practices receive enduring international interest, including the use of heroin itself as a substitute and the prescribing of methadone in an injectable form. Six linked dimensions that characterize substitution services are: (1) treatment providers and settings; (2) prescribing regimes and treatment goals; (3) substitute goals; (4) dose administration; (5) dose levels; and (6) substitute formulations. The article considers the strengths and weaknesses of the British approach and discusses key issues and future directions against a background of increasing central and local accountability for the quality and efficiency of specialist providers and critical attention to their effectiveness. Note, figures, references