In this chapter, focus is extended on the local cariogenic environment within deep stages of carious lesions. When examining various stages of deep lesions in many patients, a systematic pattern of clinical signs are exposed, that quite often are specifically related to changes of the local cariogenic environment, e.g., the undermined enamel has been broken down. When following the same carious lesion over time, a number of clinical variables can be taken into account, in particularly when carious dentin is clinically detectable, to assess lesion activity. The intra-lesion dentin characteristics over time comparing an active "closed" lesion environment (yellow/light-brown, soft and wet carious dentin) versus a slowly progressing "open" lesion environment (darker, harder and drier dentin) reflect the basic mechanism that modulates lesion activity in untreated lesions. This know-how is used as a concept during the intervention for deep carious lesions. In short, the treatment and control of deep lesions are inspired from the events taken place during natural deep caries progression and show that activity can be monitored using well-known dentin variables.