Recombinant proteins have long been used in the pharmaceutical, chemical, and agricultural industries. These proteins can be produced in hosts such as mammalian cells, bacteria, insect cells, yeast, and plants. However, the demand for recombinant proteins, especially for the prevention, diagnosis, and treatment of diseases, is increasing. Meeting this growing demand on a large scale remains a challenge for many industries. Developing new tools to increase the yield and quality of these proteins is therefore a necessity. Many strategies to optimize protein production in various expression systems have emerged in recent years. This review summarizes the different characteristics of expression systems, as well as the current strategies used to improve the yield of recombinant proteins.