As the Internet finds its way into our purses or shirt pockets, the devices we use to access it are becoming more personal too. Already today, mobile phones and PDAs know the phone numbers of our friends and colleagues. They are starting to track our location. Tomorrow, they will replace our wallets and credit cards. One day, they may very well turn into intelligent assistants capable of anticipating many of our wishes and needs, such as automatically arranging for taxis to come and pick us up after business meetings or providing us with summaries of relevant news and messages left by colleagues. But, for all these changes to happen, key issues of interoperability, usability, security, and privacy still need to be addressed.