Radicular cyst is the most common inflammatory cyst in the oral cavity. Approximately, 40% of non-keratinizing odontogenic cysts show mucous metaplasia in their linings. A case of radicular cyst with mucous metaplasia is presented with a description of the phenomenon of metaplasia and the mechanism of cholesterol cleft formation. The cystic fluid in the radicular cyst exhibits a variety of substances which may contribute to the pathogenesis of the cyst. The composition of the cyst fluid is described briefly. Radicular cysts also occur in primary dentition, accounting for 0.5-33% of the total number of cases. Differences between the radicular cysts arising from primary and permanent dentitions are considered. Certain immunochemical markers which play a key role in understanding the pathogenesis of the cyst are highlighted.