The objective of this article is to describe the various clinical situations of prolonged retention of mandibular deciduous second molars. Indications for orthodontic space closure in the absence of permanent successors and treatment alternatives in space opening, including retaining the deciduous molars, are described. Periodic monitoring, composite buildups, and indications and timing of extraction of infraoccluded and ankylosed deciduous molars with and without permanent successors are reviewed. World J Orthod 2008;9:209–220.