In this paper we present a new approach for integrating Business Process Management and Knowledge Management. We focus on the modelling of weakly-structured knowledge-intensive business processes. We develop a framework for modelling this type of processes that explicitly considers knowledge-related tasks and knowledge objects and present a workflow tool that is an implementation of our theoretical meta-model. As an example, we sketch one case study, the process for granting full old age pension as it is performed in the Greek Social Security Institution. Finally we briefly describe some related approaches and compare them to our work and draw the main conclusions and further research directions.