Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine epidemic diarrhea virus (PEDV) are two of the most prevalent swine pathogens that have impacted the global swine industry for decades. Both are RNA viruses with increasing heterogeneity over the years, making a vaccine solution ever so challenging. Modified live-attenuated vaccines (MLVs) have been the most common approach, but the long-term safety regarding their potential for pathogenic reversion still needs to be addressed. Subunit based vaccines have been the focus of numerous development studies around the world with renewed interest in their promising prospects in both safety and efficacy. Our lab has developed a unique approach to use hepatitis B virus core capsid protein (HBcAg) as a vaccine delivery vehicle for either PRRSV or PEDV viral epitope antigens. Recombinantly produced HBcAg forms an icosahedral capsid virus-like particle (VLP) that has 240 repeats in a single assembled particle. By inserting different epitope antigens from these porcine pathogens into the particle, we can achieve repetitive antigen presentation to the host's immune system by taking advantage of the polymeric nature of VLP. The first animal study evaluated the efficacy of 4 VLP based vaccine candidates against PRRSV in mice. These 4 vaccines incorporated 2 B-cell epitopes (61QAAIEVYEPGRS72 and 89ELGFVVPPGLSS100) and 2 T-cell epitopes (117LAALICFVIRLAKNC131 and 149KGRLYRWRSPVIIEK163) from PRRSV structural proteins GP3 and GP5 respectively. Candidate GP3-4 was able to stimulate a significant viral neutralizing response in mouse sera against two PRRSV strains, one being heterologous, demonstrating its potential of cross-protection against PRRSV. The second animal study took an optimized VLP vaccine candidate against PEDV from previous development studies in mice, and assessed its efficacy through a comprehensive pregnant gilt vaccination and neonatal piglet challenge model. The vaccine candidate incorporated B-cell epitope 748YSNIGVCK755 from the PEDV spike protein. It was able to elicit significant viral neutralization antibody titer in gilt milk at 3 days post-farrowing (DPF), and provided nursing piglets with clinical relief in terms of morbidity, viral shedding, small intestinal lesions, and 10 days post-challenge (DPC) survival rate. / Doctor of Philosophy / Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine epidemic diarrhea virus (PEDV) are two pathogens that infect pigs, resulting in immense economic losses to the global pork production industry every year. Both viruses have large diversity with various strains due to mutations that have occurred over the years. This makes vaccine development that aims at combating the pathogens even more challenging. One common vaccine strategy has been immunizing animals with modified live viruses with decreased pathogenicity. Naturally, long term safety of this option has been a concern. A much safer vaccine approach that is purely protein based has attracted renewed interest around the world. Protein based vaccines lack genetic materials from the viruses and are not able to replicate inside the host. Our lab has developed a platform that uses protein-based particles (VLPs) originated from the hepatitis B virus (HBV), and incorporates short pieces of proteins from either PRRSV or PEDV to train host's immune system to recognize these pathogens, and hopefully to prevent future infection. For the first animal study, we tested 4 VLP vaccine candidates against PRRSV in mice and discovered that mouse serum from one candidate GP3-4 was able to prevent infection of 2 distinct PRRSV strains in petri dishes, paving the way for further development. For the second animal study, we took an optimized VLP vaccine candidate against PEDV from previous mouse studies, and evaluated its performance in pigs. We immunized pregnant mother pigs with the vaccine before they gave birth, then experimentally infected newborn piglets with the virus. Piglets from the vaccinated mothers showed improved clinical signs and faster recovery from the infection.