This wide survey of unicameralism in sovereign countries and within federations finds that unicameralism is to be correlated with the absence of federalism, small populations and size, and not being a stable democracy. Since 1914, unicameralism has been on the rise. The predominance of unicameralism among IPU members today is explained mostly by the accession of scores of former colonies to independence in the 1960s and 1970s, while most countries that were bicameral in 1950 still are. The article discloses, however, that throughout the 1980s and 1990s far more second chambers have been created or restored than abolished, notably because many new democracies have opted for bicameralism. Case studies of democratic countries (New Zealand, Denmark, and Sweden) and subnational jurisdictions (Nebraska, Québec, and Queensland) that abolished their second chambers are provided. In all these cases, it appears that the move towards unicameralism has not been regretted, in contrast with moves towards unicameralism that occurred in non-democratic countries, which have sometimes been followed by a restoration of the second chamber.