Salmonella and Campylobacter are prominent foodborne bacterial pathogens and increased knowledge on the potential for egg transmission is beneficial when developing intervention strategies. Horizontal and vertical transmission of Salmonella into poultry flocks is well documented to occur, while only horizontal transmission of Campylobacter has been confirmed. The objective of Chapter 3 was to develop and verify injection inoculation methodology for hatching eggs that could be used for Salmonella and Campylobacter to simulate vertical transmission, in ovo. Results from these experiments established optimal inoculum levels of Salmonella Enteritidis (103 CFU) or Campylobacter coli (102 CFU) for recovery from egg contents and embryos during cold-storage or incubation. The objective of Chapter 4 was to compare Salmonella recovery from albumen or yolk inoculated eggs that were incubated or cold-stored and incubated through Day 18 of incubation. For yolk inoculated eggs samples during incubation, all egg contents were 100% Salmonella positive, and no viable embryos were present. For albumen inoculated eggs sampled on Day 18, 51% of egg contents and 37% of embryos were Salmonella positive. In Chapter 5, the objective was to compare Campylobacter recovery from albumen or yolk inoculated eggs that were incubated or cold-stored and incubated through Day 18 of incubation. Recovery of Campylobacter from egg contents of yolk injected eggs was significantly higher than for albumen inoculated eggs starting on the second day of cold-storage through Day 15 of incubation. There were no Campylobacter positive embryos recovered from the albumen inoculated eggs, but 28% of yolk inoculated eggs had positive embryos on Day 15 of incubation. However, there was no recovery from egg contents or embryos of either inoculation site on Day 18. In summary, these studies indicate that egg injection inoculation is a valid methodology for simulation and evaluation of the potential for Salmonella and Campylobacter to be vertically transmitted in ovo. For Salmonella, embryo mortality results indicate that the albumen is more likely to be the contamination site than the yolk during egg formation. Results indicate that Campylobacter vertical transmission appears to be unlikely, but if it occurs, the location of contamination is likely to be the yolk.