The highest court in the US is to decide whether or not a law to keep children away from porn on the internet is unconstitutional.
The law tries to protect children from adult content
It is the second time in two years the courts have been asked to revisit how to control unsuitable content on the net.
Part of the controversial Child Online Protection Act requires those who run websites to ask for credit card details before letting users view adult content.
But the law has continued to anger civil liberty groups who say it is censorship and violates the First Amendment right to freedom of speech.
The Justice Department have appealed to the courts again to decide whether the law places too many restrictions on what material adults have the right to see or buy.
They will also decide if the government has the right to force website operators to implement an adults-only screening system so that children do not have access to harmful material.
The court was told the law was aimed at targeting website operators who use sexually explicit "teasers" which anyone on the net can see.
These are like adverts which tempt surfers to adult sites.
Since it was passed by Congress in 1998, the law has been challenged twice and so never enforced.
In 2002, it was blocked as a result of an appeal brought by the American Civil Liberties Union, online magazine publishers, and other groups who argued it was tantamount to censorship.
A key part of the appeal was about the use of filters to block websites accessed from public libraries.
Civil liberties campaigners argued the filters were not accurate enough and blocked websites which contained information on sexual health and sexuality.
If the law was enforced, violators could face six months in prison and a $50,000 (£30,000) fine.
The Supreme Court justices will hear the arguments in early 2004 and will make their decision on the law by the end of June.