Security in grids differs from the Internet security due to the challenges that arise when we seek to build scalable virtual organizations (VOs). As discussed in Chapter 1, VOs are a group of geographically distributed individuals or organizations having permanent or temporary existence created to share resources and services among themselves. This sharing is governed by a set of rules or policies that define the conditions and extent for that sharing. The dynamic and cross-organizational nature of the virtual organization makes the problem of implementing security in grids a challenging one. The problem is complicated by the fact that there is no central point of control in grids. Absence of a central point of control in grids means that each resource provider in the grid has to make an assessment of the risk before interacting with any other service provider. To understand the process of risk assessment and subsequent trust establishment, we need to know the traditional security areas that play a vital role in defining security in the grids. These include authentication, authorization and confidentiality. You might already be familiar with these terms. However, here we explain these terms keeping in mind the grid security requirements.