Prime Minister David Cameron has promised voters that he will renegotiate the terms of Britain's EU membership before the country holds an in-out referendum by the end of 2017.
Cameron in November set out four areas of reform he wants to negotiate, which are detailed below. He wants a "legally binding and irreversible" agreement on the reforms, if needed by changing EU treaties.
If this is achieved, he has pledged to campaign "with all my heart and soul" to keep Britain in the European Union.
SOVEREIGNTY: Cameron wants an exemption for Britain from the EU's treaty-enshrined ambition of "ever closer union."
He also would like to boost the role of national parliaments, for instance by giving them the power to act in groups to stop unwanted legislation.
The EU should only take action where necessary, in line with the principle of subsidiary, while national security must remain "the sole responsibility of member states," Cameron has written.
IMMIGRATION: This area of reform is proving the most controversial, notably a proposal by Cameron that EU citizens coming to Britain should not qualify for in-work benefits or social housing until they have lived in the country and paid contributions for four years.
London argues that its public services are overwhelmed by a high number of immigrants and that it thus needs to limit the arrivals of nationals from other EU countries.
London would also like to end the practice of sending child benefits overseas, use tougher entry bans and deportation measures against fraudsters and criminals, and institute free-movement limits on nationals from countries that will join the EU in the future.
COMPETITIVENESS: Cameron argues that the EU can "go much further" on boosting its competitiveness, notably through deregulation, measures to strengthen its single market and expanded global trade links.
Britain would like to see an EU target for reducing the burden of regulation on businesses and stepped-up efforts to ensure the free flow of capital, goods and services in the bloc.
Boosting the competitiveness and productivity of the EU will help drive growth and jobs, Cameron has argued.
ECONOMIC GOVERNANCE: London wants to protect the European single market, including by guaranteeing fairness for the 19 eurozone and nine non-eurozone EU nations. Britain is not part of the single currency area and does not intend to join it.
Cameron says he is not interested in having a veto power over eurozone decisions. But he wants taxpayers in non-euro countries to not be financially liable for eurozone-support operations; eurozone decisions to never be compulsory for non-euro nations; and businesses that use currencies other than the euro not to face disadvantages.
National institutions should be responsible for financial stability and supervision in non-eurozone members and EU countries should have a say on measures that affect them all, he has argued.