The study of cell responses to protein misfolding and aggregation, boosted by the evidence of similarities between aggregation in bacteria and the formation of amyloids in eukaryotic cells, has stimulated the interest of the scientific community toward responses to protein aggregation in bacterial systems. The chapter emphasizes that even when the foreign protein is not toxic, its expression raises important physiological challenges to which bacterial cells react by activating their reaction potential toward physiological stress, which consists of different and somehow overlapping responses. Analysis of the structure of aggregated proteins is consistent with such a nonstatic view and accounts for the observation that inclusion bodies can be biologically active. Many recombinant proteins in inclusion bodies maintain their biological activity. The main advantages of in vivo immobilization include high yields, low production cost, and high tolerance to lyophilization.