Traditional views on pocket instrumentation in periodontal therapy have centred upon the thorough scaling and planing of root surfaces, which aim to remove all calculus and substantial amounts of 'necrotic' and 'contaminated' cementum. The lack of scientific evidence for removing calculus in periodontal therapy, was highlighted by Frandsen1 and recent studies have also questioned the need for extensive root planing to remove 'substantial' amounts of cementum.2,35 The need for some degree of pocket instrumentation during periodontal therapy however, is beyond doubt, as clinical studies on the efficacy of non-surgical periodontal treatment published during the last decade have clearly shown.3-6 A review of this important topic is therefore timely and this paper aims to elucidate some of the concepts which are currently the subject of debate. A resume of the formation and characteristics of the subgingival environment is a necessary starting point.