Proper periodontal diagnosis is essential for successful treatment. Hence, diagnosis should first determine whether disease is present; then identify its type, extent, distribution and severity and finally provide an understanding of the underlying pathologic processes and their cause. Hence, diagnosis may be defined as identifying disease from an evaluation of the history, signs and symptoms, laboratory tests and procedures. This chapter covers the principles of diagnosis, clinical diagnosis, and case history proforma. Periodontal diagnosis is mainly based on careful analysis of the case history and recording clinical findings with the help of various aids.