Your ISP can trivially determine that you in specific are using Tor, unless you use a bridge (preferably with Obfsproxy). Your ISP can see that you are connecting to known Tor entry nodes. They can also fingerprint your traffic as being consistent with Tor traffic, which is very unique looking. Bridges help prevent the first issue and obfsproxy helps prevent the second issue. In any case the ISP can know for sure that it is you using Tor. Tor suggests getting people nearby to use Tor to protect from a very specific sort of membership revealing attack. For example, if the DEA knows that somebody in Bobsville is sending drug shipments, then they can try to get a list of all users in Bobsville who are using Tor. If there is only one person there using Tor then they are pretty screwed, if there are a thousand people there using Tor then it is still really expensive to put them all under surveillance trying to find the one who is sending the drug shipments. I can verify that it makes no difference at all It depends. Certainly for vendors it is best if nobody can tell they are using Tor, to help prevent the attack I mentioned previously. For people in places like China they need to hide that they use Tor so that they can use Tor in the first place, since China tries to block Tor. People in some countries might even be killed if they are detected trying to circumvent the government censorship. There are a lot of situations where it is useful to hide that you are using Tor, and in general I suggest considering how important it is to you. But in some cases it doesn't really matter a whole lot. Tor naturally makes membership enumeration more difficult than it is against I2P, for example.