Despite ten years of extensive research, M. Weiser's (1991) vision of ubiquitous computing is far from a widespread reality. While research into enabling technologies has progressed significantly, there has been a lack of research into the economic and commercial aspects of ubiquitous computing. We investigate techniques that will enable investors in ubiquitous computing infrastructure and services to recoup their investment through charging for service use. In particular, we identify the key requirements for a ubiquitous computing payment system, discuss existing payment systems and present the payment session protocol (PSP) that is designed to support payment-enabled ubiquitous computing environments.