Research in our group on covalently crosslinked single-chain polymer nanoparticles (SCNPs) has previously shown consistent and monodisperse formation with particle sizes of around 10 nm. These SCNPs have the ability to incorporate various functional groups with high biocompatibility. These properties make the SCNPs an interesting nanoparticle class for biomedical applications. This dissertation focuses on using covalently crosslinked SCNPs in controlled drug delivery, with the aim of developing highly efficient nanocarriers for cytosolic delivery.