Ning Cai’s initial research topic at the University of Bielefeld was communication complexity. His PhD thesis on the 4-word property and his early joint papers with his advisor Rudolf Ahlswede were all motivated by this area. Ning’s research in this direction will be surveyed in this paper. Especially, his single-letter characterization of the communication complexity on functions defined via direct sums, later became an important tool under the name amortized complexity.