Human diseases are exceptionally complex. Consider various forms of cancer, heart disease, diabetes, atherosclerosis, diseases of immunity, genetic diseases, infectious diseases, and many others. The causes of these disease processes are equally complex and almost always multifactorial, with contributions from the host/patient (related to genetics or physiology), various exposures to disease agents, factors from the environment, and many more. The patients themselves are complex and so the presentation, consequences, and severity of disease have a tendency to vary tremendously among an affected cohort. Despite recognition of these complexities associated with human disease, researchers have historically taken reductionists approaches to their study. Hence, our knowledge of many diseases reflect the combination of results (and inferences) from ex vivo approaches, in vitro cell culture, model systems, and examination of limited numbers of molecular mediators of disease. It has been recognized for many years that the complete understanding of any disease process will require the ability to examine the condition in the context of the patient, without elimination of the complexities of the in vivo condition. Welcome to the world of systems biology and network science!