Piwik will automatically track visits on mobile phones that use browsers executing Javascript: iOs based devices such as iPhone and iPad, Android based phones or any other modern devices (running Palm Pre, Opera Mobile, Webkit, Mozilla Fennec, etc.). For low-end mobile devices that use browsers which do not execute Javascript, you can manually record visits using the Piwik Tracking API. Learn more about the Piwik Tracking API. See also Does Piwik track visitors without Javascript?