Advertisers will see you read this

[Commentary] If you feel like a shock, try finding out how many online advertising companies are tracking you every time you use the Internet. One way to do so is to go to the Network Advertising Initiative site in the US (www.networkadvertising.org) and click on the opt-out button that allows you to evade their surveillance. It also tells you how many have been watching you already. Worries about online surveillance and privacy are growing. The US Congress is investigating Internet "behavioral targeting" of this kind and Yahoo last week tightened controls on personalized advertising. Meanwhile, Phorm's planned UK trial with 10,000 BT broadband customers has caused an outcry. There is plenty to be worried about. Most people know vaguely that companies use technology to track browsing habits but are unaware of the scale and penetration of these techniques. The advent of "deep packet inspection" (DPI), practised by technology companies such as Phorm, takes this a stage further. Most of the agencies involved have adopted stricter privacy standards than in the early days of the Internet, when the abuse of "spyware" and "adware" was rampant. But there is a long way to go before those browsing the Internet can rest easy.