The core concepts, or threads, of biosystems engineering (BSEN) are variously understood by those within the discipline but have never been unequivocally defined due to BSEN's early stage of development. This makes communication and teaching difficult compared to other well-established engineering disciplines. Biosystems engineering is afield of engineering that integrates engineering science and design with applied biological, environmental, and agricultural sciences. It represents an evolution of the agricultural engineering discipline applied to all living organisms but generally does not include biomedical applications. The key element for the emerging EU biosystems engineering program of studies is to ensure that it offers essential minimum fundamental engineering knowledge and competences. A core curriculum developed by successive Erasmus thematic networks has benchmarked agricultural and biosystems engineering studies in Europe. The common basis of a core curriculum for the discipline across European countries and the U.S. has been defined by an EU-US Atlantis project, but this needs to be taken further by defining the threads that link courses together This article presents a structured approach to define the threads of BSEN. Definition of the mid-level competences and the associated learning outcomes has been one of the objectives of the EU-US Atlantis project TABE.NET. The mid-level competences and learning outcomes for each of six specializations within BSEN are defined, while the domain-specific knowledge to be acquired for each outcome is proposed. Once the proposed definitions are discussed, modified, and ultimately adopted, these threads will be available for the global development of BSEN.