There are two figures to look out for; turnout and the results.
The European elections coincide with some local and regional elections, which may help increase turnout. In 2009, the turnout of 43% (which includes several states with compulsory voting) was considered to be dangerously low.
It is being predicted that turnout this time may fall to around 38%, a figure that cause deep concern if not outright despair. 44% or above would be met with jubilation.
The first indication of the numbers of voters will be around 9pm CET on 22 May, when figures for the Netherlands should be released. The European Parliament will release turnout figures at 9pm on 25 May.
The election results are expected to be announced from 10pm CET on 25 May. However by 8pm there should be the start of exit polls, including Germany, France and Greece. There will be an estimate of the composition of the new parliament released 10pm.
Timeline
22 May - European Parliament (EP) election begins with the UK and Netherlands voting first. There are some local council elections in UK
23 May – EP election continues with Ireland and Czech Republic voting
24 May – EP Czechs end voting. Latvia, Malta , Slovakia and French Overseas Territories vote
25 May – EP Most countries vote today. Belgium also voting in regional and local elections
Ukraine, Columbia and Lithuania hold Presidential elections
27 May – European Council meets to discuss the Commission presidency and other positions. Afterwards the political groups of the European Parliament meet.
2 June – Political groups meet again, new MEPs arrive in Brussels
26 – 27 June Member states nominate candidates for Commission president
August – Member states nominate Commissioners
September – European Parliament hold hearings with nominated Commissioners
October – European Parliament vote on approving Commissioners
November – If approved, new commission takes office. Member states nominate European Council President and new High Representative for External Action Service.
December – New Council president and High Representative take office