This Special Issue of Biomedicines, "Role of NO in Disease: Good, Bad or Ugly", focuses on the roles of nitric oxide (NO) in disease from various aspects. NO plays an important role in several physiological processes with numerous biological targets, such as cardiovascular homeostasis, angiogenesis, neural signaling, inflammatory processes, and immune regulation. Hence, it is obvious that essentially all cells and biological activities are modulated by NO. This reactive and toxic two-atom gaseous molecule has an unpaired electron and is therefore considered a free radical. It is one of the most important signaling molecules. NO was most likely generated by volcanic emissions in the early atmosphere, from CO2 and N2, approximately 4.5 billion years ago, long before the appearance of the living organisms that now contribute significantly to the cycling of nitrogen species. Endogenous NO is catalyzed from L-arginine by mammalian nitric oxide synthases (NOS). However, NOS-like enzymes have also been found in procaryotes, bacteria, and eukaryotes, suggesting a basic role of NO in the evolutionary process. Thus, it is not surprising that NO signaling is spread throughout the entire phylogenetic scale.