All economic, social and legal interactions are based on certain assumptions: that individuals can verify identities; can rely on the rules and institutions governing the interactions; and are assured that certain information will remain private. These assumptions are challenged by the Web: an environment where security, privacy and trust can be very difficult to monitor, verify and enforce. Will the Web grind to a halt as a result? Will ways be found to ensure that these basic features are present? Or will users of the Web find their own ways to cope with the absence of e.g., trust? These questions call on a broad range of Web Science disciplines: to understand how individuals perceive trust and privacy when they use the Web; to see how concepts such as trust can be computationally represented; to develop the legal institutions needed to govern Web interactions. An understanding of the technology underlying security, the variables underlying trust and the extent of the privacy that Web users demand is clearly valuable in a number of industries.