The content is divided into five sections. The first introduces the reader to the concept of systems theory and its mathematical foundation. The second section provides the reader with a current view of periodontal medicine including the microbiology, molecular genetics, relationship to systemic disease and current and future therapies. Periodontitis is caused by members of the oral microbiota and the third section provides the reader with various views of the relationship of the microbiota to the host. The fourth section moves from the bacterium to the host and its immune responses to altered host: bacteria interactions. The final section deals specifically with bone destruction in periodontitis and brings the reader up-to-date with the current view of the control network that exists between mesenchymal cells such as osteoblasts, immune cells and osteoclast precursor cells that controls bone remodelling in health and disease.