So why do we get worried about complex systems and what can we do about it? Complexity worries us because the world is unpredictable, large scale, multi component and densely interconnected. We perceived interactions as complex since we have difficulty in generalising over multiple events especially when events are poorly ordered. However interactional complexity is tractable by mathematical modeling as (misnamed) chaos theory has shown. Interactional complexity is being modeled with increasing accuracy by computational theories and simulations of physical and biological systems, viz. the IPCC world climate model. The second form is semantic complexity which implicates the difficulties we have in understanding intent of people. Here sadly there is no short term tractable solution. The Dagstuhl process of discussion leading to incremental (maybe radical) advances in understanding is one answer.