Carious lesion activity assessment and caries risk assessment (CRA) are essential tools that assist professionals in proper caries management. This is to say that the diagnosis and management of carious lesions, the prescription of caries control measures and the frequency with which these measures are applied vary according to the patient’s current caries activity and caries risk profile. Moreover, they will have a direct influence on patients’ follow-up intervals. Carious lesion activity should be assessed with the Nyvad criteria which have shown prognostic value in clinical trials and which guide the professional to the most appropriate treatment decision. Assessment of caries risk is not always straightforward. Although many attempts have been made to define which factor(s) should be taken into account to better predict new carious lesions, most of the CRA tools currently available have limited validity. When a single factor is used for predicting carious lesions, the patient’s past caries experience has been shown to be the most accurate predictor. The aim of this chapter is to summarise the evidence about carious lesion activity assessment and CRA and to discuss, through providing a few clinical cases as examples, the importance of these concepts in caries management.