Five people were killed in a major terrorist incident near London's Houses of Parliament in Westminster on Wednesday.
The city held a huge terror drill just days ago - but is there any defence against the rise of the small, lone wolf attacks?
Terrorism expert and host of The Project Australia Waleed Aly joined Three's The Project to discuss whether there's any way to prevent or predict these unsophisticated attacks.
"This sort of attack, where it's an attack which emerges out of a mundane everyday activity like driving across a bridge - those attacks are extremely difficult to stop. You don't have the same warning signs," he says.
"These sorts of very low-tech crude attacks has specifically been called for by terrorist groups such as ISIS, and when you have lone wolves what they're effectively doing is a form of DIY terrorism, where people just merely latch onto an idea."
Mr Aly says that terrorism is designed to grab attention, and terrorists constantly have to thinking up new ways to shock people.
He believes there's tension between civil liberties and security.
"This is an endless tension. The reality is some level of surveillance is important, this is what intelligence agencies need to do," he says.
"The most important stuff is good relationships with the community that will then provide you with good actionable intelligence when you need it."