The party conventions held every four years by the Republican and Democratic parties are among the biggest events on the US political calendar.
Inside the venue for the Democratic Convention in Denver, Colorado
The BBC News website takes a look at what happens at the conventions, why they are held at all, and what political observers will be looking out for this year.
When and where are the conventions taking place?
The Democratic convention has been held from Monday 25 August to Thursday 28 August at the Pepsi Centre in Denver, Colorado. The Republican convention takes place from Monday 1 September until Thursday 4 September at the Xcel Energy Center in St Paul, Minnesota.
What is the official purpose of the conventions?
In the past, the parties actually selected their presidential nominees at the conventions.
But in recent decades, with the growing importance of primary elections, the presumptive nominee of each party has always been known some time before the opening of the convention.
So what happens these days?
The convention has become more of a chance for the parties to gee up their supporters, and for the candidates to set out their policy platforms in the full glow of the media spotlight.
The parties will still hold a nominal vote on the floor of the convention, but the outcomes are foregone conclusions.
It is also the function of the convention to approve the parties' official policy platforms, and to sign off on the nominee's choice of running-mate.
Again, however, these days the identity of the running-mate and content of the platforms are known in advance of the convention.
Who will speak at the conventions?
The highlight of the conventions are the final nights, when the candidates make a speech accepting their party's nomination.
In a break with tradition, Democratic hopeful Barack Obama is to deliver his speech in a separate venue from the main convention centre - he will speak at the nearby Invesco Field at Mile High Stadium.
During the preceding days, the candidates' wives and running-mates give speeches, as do prominent party officials and celebrity endorsers.
Mr Obama's wife Michelle, his running-mate Joe Biden and both Hillary and Bill Clinton spoke on the first three days of the Democratic convention.
Who attends the conventions?
Party activists who were elected as convention delegates during the primaries and caucuses earlier this year are the ones to go.
They get to participate - state by state - in the floor votes that will officially select the parties' nominees.
As well as the delegates, thousands of volunteers and journalists cover the event.
Barack Obama's acceptance speech - moved to a nearby football stadium - is open to non-delegates. More than 70,000 people are expected to attend the event.
What excitement is expected at this year's conventions?
Barack Obama is famous for his speech-making - indeed he first rose to public prominence after a much-admired speech he made at the 2004 convention.
This year, observers will be on the lookout for any hint that voters think his decision to speak in a stadium is a sign of over-confidence (John F Kennedy was the last leader who took this step).
Political watchers will also be on the lookout for any "new Obamas" - rising stars like Republican Bobby Jindal or Democrat Mark Warner, who have been given prominent speaking slots.
At the Democratic convention, speeches by Mr Obama's former rival for the nomination Hillary Clinton and her husband (and former President) Bill were much-anticipated because of previous tensions between the Obama and Clinton camps. In the end, both Clintons delivered resounding endorsements of Mr Obama.
How is the vote to confirm the nominee organised?
Usually, by the time the convention comes round all but one candidate has dropped out, and the floor vote (known as the roll call vote) becomes a celebration of the chosen nominee, as a representative from each state's delegation stands up and officially gives the nominee the state's full support.
Prior to the roll call vote, speeches are given moving and seconding the candidate's nomination.
This will be the format for the floor vote at this year's Republican convention.
How did the Democratic floor vote work this time?
As part of the "unity deal" reached by the Obama and Clinton camps, Mrs Clinton's name was placed before the delegates in the formal floor vote, as a "mark of respect" underlining just how close she came to winning the nomination.
On the day, nominating and seconding speeches were made for both Mr Obama and Mrs Clinton. The roll-call vote then began on the convention floor, with a representative from each state announcing his or her state's result.
However, part way through, Mrs Clinton stepped on to the floor, called a halt to the process and proposed that Mr Obama be nominated by acclamation - or voice vote - "in the spirit of unity, with the goal of victory".
The delegates agreed enthusiastically and Mr Obama was formally declared the Democratic Party's official presidential nominee - a historic first for an African-American.
Any resentment felt by disappointed Clinton supporters was apparently swept away in a wave of good feeling at the display of party unity.
When I think of party conventions, I think of balloons - why?
The "balloon drop" has long been a staple of the convention finale.
In 2004, the Republicans dropped some 100,000 bio-degradable balloons on to the heads of exultant conventioneers.
Why is this done? Because everybody likes balloons. Especially when lots of them are dropped from the ceiling at the same time.