The transient gene expression (TGE) in mammalian cells is an attractive option to rapidly produce research grade material with high quality that, for instance, can streamline the identification of candidate proteins in clinical trials. In this thesis we report on the performance of different methods to transiently transfect mammalian cells, in particular Chinese Hamster Ovary (CHO) cells. The used transfection methods can be categorized into physical and chemical. We compared the feasibility, the efficiency (both cost efficiency and transfection efficiency) and suitability regarding to up-scale the procedure. The optimization of the different transfection methods was carried out using statistical approaches. In this work, the successful optimization of the transient transfection and the up-scale into the bioreactor are reported. Furthermore a good insight of the different transfection methods and their parameters were obtained, which can ease the application of TGE.