Polyhydroxyalkanoates (PHAs) are accumulated intracellularly by many bacteria and serve as a carbon and energy storage compound. PHAs are polyesters of high molecular weight and can be isolated by solvent extraction and precipitation in antisolvents. The material properties of PHAs are of great interest due to the inherent biodegradability and excellent biocompatibility. To date, more than 150 different PHA monomers have been described in literature and it has been found that the monomeric unit composition significantly influences the physico-chemical properties of PHAs. The monomer composition may be controlled to some extent by the choice of the PHA production strain but also by the cultivation conditions and the carbon substrate/PHA precursor supply. In previous studies, it has been shown that the most reproducible production method of PHA is the chemostat cultivation of suitable bacteria under multiple nutrient limited growth conditions. This chapter is dedicated to provide step-by step instructions to produce PHAs in a chemostat culture and specifically describes how the composition of PHA copolymers can be tailored during biosynthesis, as well as a set of analytical tools and methods to characterize PHAs.