Oligodendrocytes (OLs) are mature glial cells that myelinate axons in the brain and spinal cord. As such, they are integral to functional and efficient neuronal signaling. The embryonic lineage and postnatal development of OLs have been well-studied and many features of the process have been described, including the origin, migration, proliferation, and differentiation of precursor cells. Less clear is the extent to which OLs and damaged/dysfunctional myelin are replaced following injury to the adult CNS. OLs and their precursors are very vulnerable to conditions common to CNS injury and disease sites, such as inflammation, oxidative stress, and elevated glutamate levels leading to excitotoxicity. Thus, these cells become dysfunctional or die in multiple pathologies, including Alzheimer's disease, spinal cord injury, Parkinson's disease, ischemia, and hypoxia. However, studies of certain conditions to date have detected spontaneous OL replacement. This review will summarize current information on adult OL progenitors, mechanisms that contribute to OL death, the consequences of their loss and the pathological conditions in which spontaneous oligodendrogenesis from endogenous precursors has been observed in the adult CNS.