© Cambridge University Press 2006. The biological production of active compounds, ranging from small molecules, such as organic acids, vitamins or antibiotics, through to macromolecules, such as therapeutic proteins or plasmid gene therapy vectors, is of great commercial and social value. The cornerstone of any such bioprocess is the cell cultivation step where a highly selected, and usually engineered, cell-line is grown under carefully controlled conditions. The term cell-lineis used here to represent both microbial and mammalian cells. The aim of the cultivation step is to yield the product in as an efficient and cost-effective manner as possible. However, the design and implementation of a cell cultivation process is often a complex, lengthy and costly task. The development of a cell cultivation process typically involves four stages, as shown in Fig. 12.1. Stage 1involves the initial identification of a native, or wild-type, cell-line that produces the compound of interest, though usually slowly and at low levels. This is followed by Stage 2in which the productivity of the chosen cell-line [g product (g cells) −1 h −1 ] is enhanced using a variety of microbiological and molecular biology techniques. Stage 3involves optimisation of the growth media composition and culture conditions; while in Stage 4, scale-up of the process from laboratory, through pilot plant to manufacturing scale occurs.