About 4,300 voting stations are open in Bosnia's two entities -- the Muslim-Croat federation and the Republika Srpska.
Bosnia's Election Commission says turnout is strong. By noon, some 25 percent of the country's 2.7 million eligible voters had cast ballots. Commission official Lidija Korac said that's double the turnout at the same time in the last elections four years ago.
MORE: Coverage in local languages from RFE/RL's South Slavic Service.
Voters will choose the three members of the collective presidency, the parliament, and the leaders of the two entities.
Reporters say, however, that the election campaign showed the continuing differences between Muslims, Serbs, and Croats, 11 years after the end of the 1992-95 war.
Polls close at 7 p.m. local time, and preliminary results are expected late in the evening.
A DEEPLY DIVIDED LAND. Bosnia-Herzegovina goes to the polls deeply divided after months of tough campaigning exposed wounds still raw from the 1992-95 war among Serbs, Croats and Muslims.
Some 2.75 million Bosnians are registered to vote for a tripartite presidency and central parliament, choosing from 36 parties, eight coalitions, and 12 independent candidates.
Voters will also choose deputies for the two autonomous regions, electing a new president, vice-president and parliament in the Republika Srpska and deputies for the assemblies of the Muslim-Croat federation and its 10 cantons.
Most Muslim parties advocate the abolition of both regions, claiming the Republika Srpska was founded on ethnic cleansing and that Bosnia can never become a viable state if one half demands a separate identity.
Croat parties want effective protection of their distinct rights, with some tentative calls to create a third entity.
Most Republika Srpska parties stand for preserving autonomy and oppose the creation of a single police force, a key European Union demand for advancing Bosnia's membership bid.
SOURCES: International Institute for Middle East and Balkan Studies, UN Development Program, Reuters