Blue cheese is any of a wide range of cheeses made with the addition of cultures of edible molds, which create blue-green spots or veins through the cheese. Blue cheeses vary in taste from very mild to strong, and from slightly sweet to salty or sharp; in colour from pale to dark; and in consistency from liquid to hard. They may have a distinctive smell, either from the mold or from various specially cultivated bacteria such as Brevibacterium linens.

A mold or mould is one of the structures that certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi. Not all fungi form molds. Some fungi form mushrooms; others grow as single cells and are called microfungi.