Bioreactors are the heart of any bioprocess operations. Currently there is a growing need for an efficient production of recombinant human therapeutic proteins worldwide. Most of the biotechnology based industries prefer Pichia pastoris as a host system for producing recombinant human therapeutic proteins because of the high cell density and product yield obtained from this particular species. This book explains the step by step approach for designing a bioreactor for producing recombinant human proteins through Pichia pastoris. The protein used in our study is only a model for this work. The novel design calculation methods presented in this book is very easy to learn and easy for the designers and students to design an efficient bioreactor for producing any proteins or enzymes in an industrial scale by only changing the few variables such as the empirical formula of the microbe and product of interest, operating conditions etc.