Collaboration has become a dominant feature of modern science. Many scientific problems are beyond the realm of individual discipline or scientist to solve and hence require collaborative efforts. Meanwhile, today's science becomes increasingly more dataintensive, resulting in a rapid transition from computational science to e-Science (or digital science). Recently, scientific workflows have emerged for scientists to integrate distributed computations, datasets, and analysis tools to enable and accelerate scientific discovery. The convergence of the above two trends naturally leads to the concept of collaborative scientific workflows. This paper presents a disciplinary definition of this term, discusses the opportunities, requirements, and challenges of collaborative scientific workflows for the enablement of scientific collaboration, and concludes with our ongoing work in this direction.