This paper deals with fluctuations over time in the extent of the state-owned firm sector nationalization-privatization cycles (NPCs). Anecdotal observations of NPCs are supported threefold by systematic empirical evidence: first, for different regions and economic systems, second, for different time periods, and third, for different indicators. In explaining NPCs, the paper provides a history of research on the subject. At the end three hypotheses are distinguished: the ideological preference hypothesis, the political cycle hypothesis, and the efficiency hypothesis. The conclusion is that NPCs existed in the 20th century as a worldwide phenomenon and are worthy of being explained.