The Grid-based infrastructure enables large-scale scientific applications to be run on distributed resources and coupled in innovative ways. However, in practice, Grid resources are not very easy to use for the end-users who have to learn how to generate security credentials, stage inputs and outputs, access Grid-based schedulers, and install complex client software. There is an imminent need to provide transparent access to these resources so that the endusers are shielded from the complicated details, and free to concentrate on their domain science. Scientific applications wrapped as Web services alleviate some of these problems by hiding the complexities of the back-end security and computational infrastructure, only exposing a simple SOAP API that can be accessed programmatically by applicationspecific user interfaces. However, writing the application services that access Grid resources can be quite complicated, especially if it has to be replicated for every application. In this paper, we present Opal which is a toolkit for wrapping scientific applications as Web services in a matter of hours, providing features such as scheduling, standardsbased Grid security and data management in an easy-to-use and configurable manner.