BPEL is the de facto standard for business process modeling in today's enterprises and is a promising candidate for the integration of business and scientific applications that run in Grid or Cloud environments. In this paper, selected components of BPEL4Grid, a scientific workflow system for developing service-oriented Grid applications based on BPEL, are presented. The focus of the paper is on security aspects of workflow composition and fault tolerance mechanisms for long-running workflows. Furthermore, two workflow modeling tools targeted at users with different levels of knowledge about BPEL are briefly described.