Inter- and transdisciplinarity are increasingly relevant concepts and research practices within academia. Although there is a consensus about the need to apply these practices, there is no agreement over definitions. Building on the outcomes of the first year of the COST Action TD1408 “Interdisciplinarity in research programming and funding cycles” (INTREPID), this paper describes the similarities and differences between interpretations of inter- and transdisciplinarity. Drawing on literature review and empirical results from participatory workshops involving INTREPID Network members from 27 different countries, the paper shows that diverse definitions of inter-and transdisciplinarity coexist within scientific literature and are reproduced by researchers and practitioners within the network. The recognition of this diversity did not hinder the definition of basic requirements for inter- and transdisciplinarity. We present five basic units considered as building blocks for this type of research. These building blocks are: (1) creation of collective glossaries, (2) definition of boundary objects, (3) use of combined problem- and solution-oriented approaches, (4) inclusion of a facilitator of inter-and transdisciplinary research within the team and (5) promotion of reflexivity by accompanying research. These were considered five basic units for effective inter- and transdisciplinary research although the 4th building block was also considered as “matrix” that holds all the others together.