After yesterday's largely fruitless meeting with French President Francois Hollande in Paris and a tough breakfast with his Polish counterpart this morning, David Cameron heads to Berlin for lunch with Angela Merkel.
The meeting with the German Chancellor will be the most important of his whistle-stop tours to European capitals this week as he tries to make a head-start in his bid to renegotiate Britain's relationship with the European Union.
He will tell Ms Merkel the current arrangements with the EU are "not good enough" and lay out the arguments in favour of EU-wide reform.
Win over Ms Merkel - the driving force of the EU and by far the most powerful figure of the 28-state bloc - and Mr Cameron makes a significant step towards achieving meaningful reform that is likely to convince the British public to vote in favour of staying in the EU in a referendum that could be held as early as next year.
But what exactly are the reforms the Prime Minister will be trying to persuade Ms Merkel to agree to?
1. Benefit tourism
The toughest challenge of Mr Cameron's efforts to secure meaningful reforms to Britain's relationship with the EU is his call to restrict EU migrants' access to Britain's welfare system.
The free movement of people is a fundamental principle of the EU but it has led to soaring levels of immigration to the UK as a relatively strong economy has attracted workers from struggling European economies - a phenomena that the UK government has no control over.
Mr Cameron, instead of choosing the impossible path of trying to crack down on one of the founding principles of the EU - freedom of movement - has opted for the more realistic move to tackle EU migration by limiting the appeal of Britain's welfare system to foreigners.
He wants to do this by blocking EU migrants' access to tax credits and child benefit until they have lived and contributed to tax revenues for four years; restrict their access to council housing; ban EU migrants from sending child benefit or child tax credit abroad, block EU migrants from claiming jobseekers' allowance and deport any EU migrants who have not found a job within six months of arriving in the UK.
Mr Cameron has insisted that these changes to welfare to cut EU migration "will be an absolute requirement in my renegotiation" and wants the reforms to be enshrined in new treaties. Ms Merkel, Ms Hollande and other major EU leaders are resistant to treaty change however, and Eastern European states are set-against Mr Cameron's bid to block access to the welfare state. European leaders argue his proposals would contravene the principle of freedom of movement.
2. Repatriating powers to national parliaments
He wants national parliaments to be given the power to block EU legislation, thereby restoring the UK Parliament's sovereigntly and giving MPs the power to say no to "unwanted" European laws. This is a move welcomed by a host of fellow European leaders but Ms Merkel will be more resistant as it will represent a significant step-back to European integration.
3. An opt-out of 'ever closer union'
Mr Cameron wants Britain to be exempt from one of the key principles of the EU - the goal of moving towards an "ever closer union".
The principle was laid out in the 1958 Treaty of Rome - before the UK joined the European project - and Mr Cameron is likely to face opposition from countries such as France, who will point out that the wording refers to an "ever closer union among the peoples of Europe" and not the 'states of Europe'.
4. Cut back red tape
Mr Cameron wants to free businesses up from the "excessive interference" of the EU's bureaucratic rules and regulations that he claims holds back enterprise and efficiency. He will have no trouble convincing Ms Merkel in this area of reform.
5. Strengthening the single market
In tandem with cutting back on red tape, Mr Cameron wants to break down trade barriers by opening up new markets for EU businesses. He is calling for EU leaders to "turbo charge" free trade deals with mass markets in America and Asia, as the current free trade negotiations with the United States and South Korea are attempting to achieve. This is a move welcomed by Ms Merkel.
6. Protection against changes to the Eurozone
Mr Cameron will push for safeguards to protect Britain against the further integration of the Eurozone. He wants to ensure the currency bloc does not undermine the integrity of the Single Market, which may disadvantage the UK. This could prove troublesome with Ms Merkel, who is keen to push ahead with wide-ranging reforms to the Eurozone without enacting treaty change.
7. Financial protection for the City of London
The Conservative party manifesto promises to safeguard Britain "as a global centre of excellence in finance" and will do so by resisting attempts by the EU to restrict "legitimate financial services activities".
This move may prove tricky as Ms Merkel and Mr Cameron have not exactly seen eye-to-eye with previous EU plans to move towards a fiscal union, that the Prime Minister warned would hit Britain's financial sector. The German Chancellor has to balance UK calls for less regulation with demands to reform the Eurozone, which requires more fiscal rules.