Water-soluble, nano-sized crosslinked polymer networks, or nanogels, are delivery vehicles, which have highly interesting properties for therapeutic delivery and imaging. Nanogels may also possess responsive properties, depending on the employed polymers, allowing controlled release of therapeutics or image contrast generation upon exposure to physical or (bio)chemical cues. In this review, polymer nanogels are explored for application in imaging as well as for controlled drug and gene delivery. Moreover, nanogels are explored as responsive biomaterials and future applications are highlighted.