Uncertainty is an issue all modellers and modelling frameworks must work with. In the UncertWeb project (www.uncertweb.org) we have been developing tools and methods to help modellers address the issue of uncertainty in the context of web based workflows. In this workshop we will provide an overview of the vision of UncertWeb and its other substantial achievements (UncertML, information models, workflow tools). We will then run a hands-on session for users to explore the UncertWeb tools including tools to allow the creation and execution of web-based model workflows, the expert elicitation of uncertainty, the sensitivity analysis and emulation of models exposed on the web, and the visualisation of uncertainties in the outputs from models and workflows. We will finish with a summary of the state of the art and open issues.