The study will be a single centre, examiner masked, two arm, split mouth, randomized controlled clinical trial, to evaluate the clinical effectiveness of mechanical cleaning (RSD) plus additional use of a light activated dye (photodynamic therapy-PDT) compared to mechanical cleaning alone, in the treatment of residual pockets, in patients with chronic gum disease. The null hypothesis is that there is no difference between RSD and the additional use of PDT compared to RSD alone, when treating patients with chronic gum disease. The local delivery treatment of PDT requires the low power light to be shone into the pocket for 1 minute in order to activate the dye which kills the bacteria. For this reason it is a treatment which is more suited to treating localised rather than widespread disease. Thus, patients with chronic gum problems who have had a cycle of treatment with the hygienist, but have a few residual pockets will be selected for this study. Subjects will be recruited from patients attending the hygienist following a first cycle of RSD, who have sufficient remaining disease to represent a meaningful challenge. This reflects the normal care pathway for patients, where simple approaches to treatment are followed up with more complicated therapy for any areas which fail to respond. A power calculation has shown that 29 patients are expected to be enough to see if PDT \& RSD can produce better gum health than RSD alone. However, to account for unexpected patient withdrawals, 35 patients will be recruited to this study. The staff hygienists in the department are capable of providing treatment for 60-90 patients over a 3-month period. Out of these patients those with more severe disease are likely to have residual pockets that require further cycles of RSD. Thus there is potentially a good population of patients attending Charles Clifford Dental Hospital to recruit from. All residual sites within a subject who participates in the study will be treated by mechanical cleaning. This is the standard treatment which would apply for most patients in this situation. A random allocation procedure will be used to designate sections of the mouth to receive additional treatment with the PDT. This will not be made known to the operator performing the RSD until after they have completed that aspect of treatment. This helps reduce bias as the operator will clean all sites to an equivalent standard. The split mouth approach means that the patient will act as their own control. This helps reduce the numbers of patients which need to be recruited as compared to a study with 2 parallel groups, where each group receives a different treatment. The suggested disadvantage of a split mouth design, is the fact that there may be some contamination of RSD sites only with the action of a chemical remote from the site it was originally placed in. However, in this study the chemical needs to be activated by the light, and therefore it is easier to control the activity of PDT therapy to the sites it was designated to be used in. For this reason there needs to be a gap between the section of the mouth treated by adjunctive PDT \& RSD, compared to the section of the mouth where RSD alone is the treatment. No additional appointments will be required of the patients compared to the normal protocol as they will receive the "gold standard" treatment of mechanical cleaning (RSD) in all sites. The additional requirements of the study will be the time it takes to provide treatment using PDT. This is anticipated to be between 5-15 minutes depending upon the number and distribution of sites throughout the mouth. Sample collection of bacterial plaque and gum fluid from 2 sites per patient will take an additional 5-10 minutes. The normal treatment time at this stage of treatment would be 60 minutes and this will be extended to 70-85 minutes for the study purposes. None of the additional procedures are likely to cause discomfort. The 3 month re-assessment examination will require repeat sample collection of bacterial plaque and gum fluid from 2 sites which will take an additional 5-10 minutes. Thus the standard patient reassessment appointment of 60 minutes will be extended to 65-70 minutes. The full process from recruitment of patients to writing up this student project is expected to be finished within seven to nine months.