The degradation of mortars and plasters can represent a threat for their preservation. That is why a deep understanding of the degradation mechanisms and the identification of degradation patterns is strongly recommended for who is in charge of conservation of archaeological sites and of built heritage in general. Here, it has been described what are the main degrading agents that can be detected on mortars and plasters and how they act to produce degradation. Moreover, an overview of the analyses which can be carried out directly on site and in laboratory as well has been reported. The knowledge that can be achieved by using such methods represents an essential tool to set up a suitable conservation plan. In addition, a more detailed analysis can also have a research purpose, since they can be useful to clarify some mechanisms and interactions that still remain unclear.