Anyone who has participated in the blogosphere in the past two months knows the troubling story of Kathy Sierra, a prominent blogger who was the victim of online threats that included violent sexual acts and murder. When the harassment spread beyond her own blog to two others that were affiliated with other prominent bloggers, Sierra became so terrified that she canceled an upcoming speaking engagement and took a hiatus from blogging.

But Sierra isn't the only one to endure online harassment. In fact, some would argue that she's just the most visible -- if not the most historically egregious -- tip of an iceberg that has been around since Internet discussions began in the early 1980s. "Between now and the early days of Usenet, the level of abusive behavior has been distressingly constant," says Tim Bray, a veteran blogger and director of Web technologies at Sun Microsystems.

The difference is, with 70 million blogs in existence today and 1.4 new blogs created every second, according to blog search engine Technorati, there are just more people participating in online discussions, and "the more crazy people you've got reading them, the wilder the whole blogosphere can become," says Richard Silverstein, who advocates for a peaceful approach to solving the Israeli-Palestinian conflict in his blog.

And he should know. Like Sierra, Silverstein is the victim of online harassment, in the form of hostile comments on his own blog, in external discussion groups and on blogs created solely for the purpose of maligning him. Given the topic that he blogs about, Silverstein is no stranger to abusive commentary. "It's part of the territory -- if you want to write a blog like this, you're going to deal with unpalatable people," he says.

But when the external blogs -- whose creators were anonymous -- grew increasingly threatening, including what he saw as pornographic photographs, he began to feel personally harassed. "I've felt insecure and under threat," he says. "No one has said, 'I'm going to come and kill you,' but there were some comments that got me concerned. You hate to think of these things, but it's very possible that some wacko will escalate from a threatening comment to actually doing something."

Silverstein has been able to uncover the identities of the bloggers, but he's been unable to force the blogs' removal, despite repeated correspondence with Blogger.com, which cites Section 230 of the Communication Decency Act that shields providers of content creation tools from liability for the content users create. In an e-mail sent to Silverstein, Blogger.com said that the site "does not remove allegedly defamatory, libelous or slanderous material from Blogger.com or BlogSpot.com," pursuant to Section 230, although it did remove the photographs because they were copyrighted images.

While both Silverstein and Sierra are higher-profile bloggers than many of us, it's clear that anyone who enters the blogosphere needs to be aware of the types of people who get satisfaction out of online harassment. According to Derek Wood, vice president of clinical operations at PsychTracker, a journaling site for people with mental illness, the harassment comes in two general forms: trolls and cyberstalkers. It's important for blog participants to understand the psychological makeup of both types so that if they encounter any type of online abuse, they'll have some idea of what they're facing and how to respond to it.