In research projects that we are currently conducting, we cooperate with researchers from a number of Polish, Norwegian and British institutions: the Institute of Psychology of the Polish Academy of Sciences, Oslo Metropolitan University (OsloMet), Universitetet i Oslo, Inland Norway University of Applied Sciences, University College London, Lancaster University and University of Cambridge. We also have experience in cooperation in international COST networking programs: A33, IS0804, IS1306, IS1406. International cooperation is important for our team because studies of bi- and multilingual language development should include both languages of the child and the two languages should be analyzed in a similar manner. We co-created and still co-create various tools for assessing language in multilingual children: LITMUS-CLT, LITMUS-SRep, LITMUS-MAIN. We use these tools in our own studies and lend them to other researchers for scientific purposes. We are currently carrying out two research projects with external funding: StarWords (NCN - OPUS programme) and PolkaNorski (NCN - GRIEG programme, financed by the Norway Grants). We also co-create Action II.3.7 "Multilingualism" under the strategic program of the University of Warsaw: Excellence Initiative - Research University and we participate in another strategic program of the University of Warsaw: Alliance 4EU+, under which we participate in the European Network for Psycholinguistic Research on Multilingualism and Multilingual Development.