Complex coacervation between therapeutic proteins and hydrophilic ionic-neutral block-copolymers leads to the formation of core-shell structured nanoparticles gently packaging the proteins in its core (reviewed in CHAPTER 1). This kind of nanoparticle, termed block ionomer complex (BIC), has great potential as protein delivery vehicles because of its simple and non-denaturing manufacturing procedure. However, reports on in vivo application of protein BICs are not common, primarily because of their instability at physiological ionic strength. The focus of this thesis is to explore the usage of BIC as an in vivo protein delivery vehicle. Our lab previously developed a protein BIC formulation, “SOD1 nanozyme”, formed between the protein superoxide dismutase 1 (SOD1) and the block copolymer poly(ethylene glycol)-poly(L-lysine) (PEG-PLL) followed by crosslinking with 3,3'-dithiobis(sulfosuccinimidyl propionate) (DTSSP). CHAPTER 2 examines the mechanism for SOD1 nanozyme to be effective for stroke treatment. Active incorporation of SOD1 nanozymes into the growing thrombus turns out to retain them at the vicinity of the injured sites on blood vessels after stroke. Although helpful in the retention of SOD1 nanozymes after stroke, polylysine can be toxic in vivo because of their cationic charge which easily disrupts cellular membranes. CHAPTER 3 describes a project that aims to replace the polylysine component with poly (aspartate diethyltriamine) (PAsp(DET)). This polymer is less toxic than polylysine due to its unique two-step protonation behavior, and its BIC with SOD1 has a similar size as the PLL-based SOD1 nanozyme. The therapeutic efficacy of this new formulation is also close to the PLL-based formulation. This dissertation also involves the characterization of a protein BIC formulation without crosslinking. We found that the BIC formed by brain derived neurotrophic factor (BDNF) and PEG-poly (L-glutamic acid) is stable at physiological salt concentrations. While protecting the cargo BDNF from interaction with a variety of mucosal proteins, the complex specifically releases active BDNF in the presence of its receptor, tropomyosin receptor kinase B (TrkB). Compare with native BDNF, The complex delivered significantly higher amounts of protein to different brain regions after intranasal delivery. This work is presented in CHAPTER 4. In summary, BIC is a promising platform for in vivo delivery of therapeutic proteins with careful design of stabilization strategies.