Avian Avulaviruses (formerly Avian Paramyxoviruses, APMV) are important pathogens of avian species and have been used as viral vectors for more than two decades. Among all APMVs, Newcastle disease virus (NDV or APMV-1) has been most extensively used as a vaccine vector for protection against avian and animal diseases, and as an oncolytic agent. For poultry vaccination, the preexisting maternal antibodies against NDV can neutralize APMV-1 vectors resulting in vaccination failure. Hence, there is a need to develop new vaccine vectors that would escape neutralization by the maternal antibodies. In the first part of my study, I created a reverse genetics system for Avian paramyxovirus-3 strain Wisconsin (APMV-3 Wisc.) which was proven to be nonpathogenic for day-old chicks and embryonated chicken eggs. The virus was used as a vector to express the enhanced green fluorescent protein (GFP) as a heterologous antigen. The recombinant APMV-3 Wisc. expressing GFP was compared with similarly constructed APMV-1 strain LaSota and APMV-3 strain Netherlands-based vectors for GFP expression and growth kinetics in vitro, and for immunogenicity, safety, and tissue tropism in day-old specific pathogen-free (SPF) chicks. APMV-3 strain Netherlands (APMV-3 Neth.) showed the highest growth rate and GFP expression in chicken fibroblast DF-1 cells, followed by APMV-1 LaSota and APMV-3 Wisconsin. In day-old chicks, APMV-3 Neth. spread to different organs, decreased feed intake and caused stunted growth. APMV-3 Wisc. and APMV-1 LaSota were confined to the respiratory tract and did not induce any pathogenic effects. All three constructs induced seroconversion of the vaccinated chicks for the vector antigens. Thus, the reverse genetics system created in this study for APMV-3 Wisc. allows the development of safe APMV vector antigenically different from NDV that can be used for day-old chicks vaccination. In addition, it provides a tool to study the molecular basis of APMV3 pathogenesis. In the second part of my study, I explored a novel approach for the expression of a foreign gene as an uninterrupted open reading frame (ORF) with a cognate gene of NDV vector. This approach is expected to promote the foreign gene expression stability. Avian influenza virus (AIV) hemagglutinin (HA) protein-coding sequence was fused in-frame with various proteins of NDV vector, with a 2A self-cleaving peptide, a furin cleavage site, or both, placed between the AIV and NDV sequences for separation of the two proteins. Among different constructs tested, we only recovered viable viruses with AIV HA fused C-terminally to the NDV HN gene. These viruses demonstrated a higher expression level of AIV HA than the vector constructed according to a traditional scheme of expressing the transgene as a separate transcriptional unit. Also, they showed increased stability of the transgene expression over multiple passages in embryonated chicken eggs. Our results demonstrate the advantages and limitations of this novel method of foreign gene expression that need to be considered for the development of NDV-based vaccine or therapeutic vectors.