Study Design: This is a Phase 1/1b, open-label study to examine safety, tolerability and immunogenicity of investigational Ebola vaccines in healthy adults. Part 1 is a Phase 1 dose escalation of the cAd3-EBO vaccine that encodes wild type (WT) glycoproteins (GP) from Zaire and Sudan strains of Ebolavirus. Part 2 is a Phase 1b further evaluation of the cAd3-EBO vaccine at the highest dose and evaluation of the Zaire component, which will be provided as a vaccine designated cAd3-EBOZ. The hypotheses are that the study vaccines, cAd3-EBO and cAd3-EBOZ, will be safe and will elicit immune responses to Ebola GP. The primary objectives are to evaluate the safety and tolerability of the study vaccines administered as single intramuscular (IM) injections at two dose levels. The secondary objectives are related to evaluation of the immunogenicity.