Visualization has a relevant role in almost every domain of computer applications. It is thus natural to think about bringing visualization techniques to mobile devices (such as PDAs and mobile phones) to harness the power of visualization anytime, anywhere. Unfortunately, limitations of mobile devices make it impossible to follow a trivial porting approach from desktop computers. A considerable research effort is needed to understand how to design effective visualizations for mobile devices. This paper deals with the different aspects of visualizing information on mobile devices. We first discuss in detail the peculiarities of the mobile visualization context that motivate research needs. Then, we summarize the different steps of mobile visualization design. We provide concrete examples from a mobile application we have recently developed and also present a taxonomy of the different classes of visualizations that are being investigated in the mobile context.