Both processes have their shortcomings.  Blacklisting will always struggle to keep up-to-date with the dynamic nature of the Internet, where old content is frequently moved and new material constantly created. 

Filtering has had the opposite problem of identifying websites unfairly.  Commonly cited examples include websites dealing with the subject of breast cancer or homosexuality.

The companies involved are at pains to point out that filtering techniques are constantly being developed.  Trevor Rushka, International Marketing Director at Finnish company Codewise said:

"The responses from users, as well as working with experts and cultural specialists have helped improve performance."

No filter software claims to be perfect.  The website of CyberPatrol, one of the best-known products on the market, says filtering software is best used as "a companion to good parenting and educating children about Internet safety rules."

Filtering software offers an increasing number of options for parents.  Many products allow parents to add or remove websites to a blacklist.  It is also now possible to monitor what has been accessed, or limit how long can be spent on particular sites.

Surveys suggest some confusion about families' use of filters.  In the recent UK Children Go Online (UKCGO) survey 46% of parents claimed they had filter software installed.  However 26% admitted they were not sure whether they had filters or not.  And 20% did not know how to install the software themselves.