Do I have to host all sites that are tracked?

Do I have to host all sites that are tracked? I understand I have to load Piwik on my server to track, say, my own web site or sites I host. But what if I want to track a client site that is hosted else where?

Do I have to load Piwik on their server as well?

Thanks in advance. I searched for this but got no answer.

See FAQ: Can I install Piwik on one server and track visits on websites that are hosted on other servers?