The Underground Built Heritage (UBH) class, introduced at a methodological and theoretical level in 2021, collects cultural assets excavated throughout history from underground that, today, are significant elements of local cultural heritage, having strongly impacted on the “sense of place” of local communities. Those cultural assets have been at the core of several national and international regeneration processes, and, recently, new tools have been developed to support their classification and sustainable reuses. This entry introduces tools for the classification of historical uses of UBH elements and their historical reuses, as well providing guidelines for their sustainable regeneration for the social and economic benefit of local populations based on the analysis of some worldwide case studies.