In this paper, we present SeaLion, an integrated development environment (IDE) for answer-set programming (ASP). SeaLion provides source-code editors for the languages of Gringo and DLV and offers popular amenities like syntax highlighting, syntax checking, code completion, visual program outline, and refactoring functionality. The tool has been realised in the context of a research project whose goal is the development of techniques to support the practical coding process of answer-set programs. In this respect, SeaLion is the first IDE for ASP that provides debugging features that work for real-world answer-set programs and supports the rich languages of modern answer-set solvers. Indeed, SeaLion implements a stepping-based debugging approach that allows the developer to quickly track down programming errors by simply following his or her intuitions on the intended semantics. Besides that, SeaLion supports ASP development using model-driven engineering techniques including domain modelling with extended UML class diagrams and visualisation of answer sets in corresponding instance diagrams. Moreover, customised visualisation as well as visual editing of answer sets is realised by the Kara plugin of SeaLion. Further implemented features are a documentation generator based on the Lana annotation language, support for external solvers, and interoperability with external tools. SeaLion comes as a plugin of the popular Eclipse platform and provides interfaces for future extensions of the IDE.