Bioreactors are normally used for a specific experiment or production run—which may last for hours, days, or even weeks in a row, depending on the utilized organism and intended product. Sequential steps of bioreactor preparation, medium formulation, and preculture/inoculation are required and, as a whole, they constitute upstream operations; cultivation proper comes next as onstream operation, involving physicochemical and mechanical tools; these are complemented by cell harvest, followed by extraction/separation/purification of cells or compounds—which together account for downstream operations. This book focuses on up- and onstream operations since they are crucial to functioning of any bioreactor and ultimately leverage or constrain its performance.