This article reviews the diagnostic process, from the first clinically evident stages of the caries process to development of pulpal pathosis. The caries diagnostic process includes 4 interconnected components–staging caries lesion severity, assessing caries lesion activity, and risk assessments at the patient and tooth surface level - which modify treatment decisions for the patient. Pulpal pathosis is diagnosed as reversible pulpitis, irreversible pulpitis (asymptomatic), irreversible pulpitis (symptomatic), and pulp necrosis. Periapical disease is diagnosed as symptomatic apical periodontitis, asymptomatic apical periodontitis, acute apical abscess, and chronic apical abscess. Ultimately, the goal of any diagnosis should be to achieve better treatment decisions and health outcomes for the patient.