Book excerpt: Periodontal disease is caused by bacterial plaque and is the most frequent reason for tooth loss in adults. It is now well-recognized that certain systemic diseases can affect the response of the periodontal tissues to plaque. Furthermore, certain types of systemic drug therapy can have an adverse effect on the gingival tissues, whereas other drugs with antimicrobial and anti-inflammatory actions may be beneficial to the periodontal tissues. All of these problems are encountered in dental practice, especially those specializing in periodontology. The aim of this book is to document and discuss the effect of various systemic factors and drugs on the periodontal tissues and their response to plaque. This book is aimed at the periodontologist or dental practitioner with a specialty in periodontology. The book is also a useful reference for postgraduate students.