US President Barack Obama has lifted restrictions on federal funding for research on new stem cell lines.
Government funding had been blocked since August 2001 by his predecessor, George W Bush.
Scientists say stem cell research will lead to medical breakthroughs, but many religious groups are opposed.
An emotional debate has raged between those who say the research is vital for medical progress and those who say it destroys human life.
The move may have implications for a separate ban on federal funding for human embryo experiments.
What are stem cells?
They are the body's master cells, the source of all cells and tissue, including brain, blood, heart, bones and muscles.
Embryonic stem cells come from days-old embryos and can produce any type of cell in the body.
Scientists generally harvest embryonic stem cells from embryos left over after in vitro fertilisation attempts at fertility clinics but they can also be produced using cloning technology.
The scientific hope is that the transformational qualities of stem cells can be harnessed to treat diseases ranging from injuries to cancer.
When will the money flow?
Mr Obama has given the US National Institutes of Health 120 days to formulate guidelines on federal stem cell research.
Why was President Bush against it?
While he was a strong supporter of adult stem-cell research, Mr Bush argued that using material from human embryos was very different.
Vetoing a Democratic bill in June 2007, he said that to allow it through would be to "compel American taxpayers... to support the deliberate destruction of human embryos".
He exhorted scientists to "pursue the possibilities of science in a manner that respects human dignity and upholds our moral values".
How does Mr Obama defend it?
The new president described himself as a man of faith who had carefully weighed the implications of the decision.
He said that scientists had been deserting the US for other nations and "medical miracles", with the potential to help victims of debilitating illnesses and catastrophic injuries, could only be achieved through painstaking research.
"When government fails to make these investments, opportunities are missed," he said.
Will experiments on human embryos be affected?
A ban remains in place on federal funding for the creation or destruction of human embryos.
Known as the Dickey-Wicker amendment, the ban has been in place since 1996 and comes up before Congress for renewal every year.
Analysts say Mr Obama's decision may lead Congress to overturn the ban - something unthinkable under President Bush.
The creation of embryos is a routine practice in private fertility clinics.
Why is the debate so politically charged?
Religious and social conservatives who oppose abortion, consequently oppose the destruction of embryos.
High-profile advocates of stem cell research have helped thrust the debate into the spotlight.
The death of former Superman actor Christopher Reeve - a prominent advocate - generated great interest in the issue.
Campaigning by other famous names like Michael J Fox and Nancy Reagan has also kept it in the public eye.
Mrs Reagan argues that treatment of Alzheimer's - the disease from which her late husband, President Ronald Reagan, suffered - could be helped by stem cell research.