BACKGROUND There is ample evidence for human alteration of Europe’s regional seas, particularly the enclosed or partly enclosed Baltic, Black, Mediterranean, and North Seas. Accounts of habitat and biodiversity loss, pollution, and the decline of fish stocks in these economically, socially, and ecologically important seas demonstrate unsustainable use of the marine environment. At the same time, there is an insufficient quantity and quality of information to enable purely evidence-based management of Europe’s seas despite this being a declared goal of many decisionmakers; for example, less than 10% of the deep sea has been systematically explored (UNEP 2006). Evidence-based management alone is rarely possible in situations with complex value-laden policy options (Greenhalgh and Russell 2009), and unfortunately, many of the most pervasive problems in the marine environment are “wicked” second-order problems (Jentoft and Chuenpagdee 2009): they are complex in nature and their management will often involve both winners and losers. Solutions to these problems involve less politically attractive, valuebased choices and may require long time lags before tangible results are observed. Fisheries management, habitat and species protection, competition for marine space, and invasive species are all examples of “wicked” problems. These are some of the biggest issues facing Europe’s seas and are the major focus of this article and Special Feature. For the first time in European history, most countries have adopted a common maritime policy (the 2007 Integrated Maritime Policy) and a legally binding environmental directive (the 2008 Marine Strategy Framework Directive [MSFD]). These comprehensive policy vehicles encompass, or closely interface with, more specific measures, such as the recently reformed Common Fisheries Policy, the Water Framework Directive, the Habitats and Birds Directive, and a number of targeted policy instruments that deal with aspects of pollution control and coastal zone management. The overall array of measures has the potential to ensure the sustainable use of Europe’s seas and the restoration of marine environments, but the pathway between the current situation and the implementation of an ecosystem approach to management (the aspiration of the European Commission; see Our Approach to Research) is fraught with “wicked” problems. Science can help society resolve these problems, but in many cases this requires the broad and integrative vision of Odum’s (1971) “macroscope” rather than trying to piece together an ill-fitting jigsaw puzzle of discipline-focused information. This paper and the others in this Special Feature employ a systems approach. We describe the approach, how it can be applied practically, and some of the challenges in making it work. Though the work is based on research on Europe’s seas, it has much wider implications for regional seas throughout the world. OUR APPROACH TO RESEARCH ON MARINE SOCIALECOLOGICAL SYSTEMS The research described in this paper (and Special Feature) was conducted in the framework of the EU-FP7 funded project Knowledge-based Sustainable Management of Europe’s Seas (KnowSeas). The interdisciplinary research spanned 4 years and involved 33 institutions from 16 European countries (KnowSeas 2013). Its primary objective was to develop “a comprehensive scientific knowledge base and practical guidance for the application of the ecosystem approach to the sustainable development of Europe’s regional seas.” Given the knowledge gaps and uncertainties in the way Europe’s marine social-ecological systems function (e.g., unresolved causal links, poorly mapped habitats, nonlinear dynamics), an iterative approach to inquiry was adopted, based partly on the reasoning behind soft systems analysis (e.g., Checkland 2000).