Diagnosis of disease is a process of hypothesis, investigation, and synthesis. Regardless of whether a human, animal, or plant is afflicted, the process of diagnosis is strikingly similar. Positioned on the biosecurity continuum between surveillance and response, early and accurate diagnosis is critical to effective mitigation and management of disease. Infectious diseases have the potential to spread among animal or plant populations, jump species barriers, and result in epidemics and global pandemics. Additionally, zoonotic infectious agents can also significantly impact human health on a mass scale. It is critical that infectious diseases be identified and detected in a timely fashion to prevent spread. This chapter will delve into the resources and supporting activities for that process, demonstrated via case studies from animal and plant systems, illuminating similarities and differences in the diagnostic process tools that can be mobilized and enhanced for biosecurity.