Nitric oxide (NO) is known as an unstable signaling molecule that can be produced by three different NO synthase (NOS) isoforms. It plays a vital role in a wide range of physiological processes in the body. For instance, in cardiovascular system NO acts as a blood vessel relaxant, while in central nervous system (CNS) it acts as a neurotransmitter. In reproductive system it regulates gonadotropin hormone, oocyte maturation, ovulation, movement of fallopian tube, contraction of uterus during labor, capacitation of sperm, erection, and ejaculation. It has been reported that NO regulates secretion, absorption and motility of gastrointestinal system. It also plays a significant role in the whole process of inflammation and dynamics of Ca⁺² muscles are regulated by NO concentrations. This gas also relaxes the blood vessels and airways of respiratory system. It regulates angiogenesis, apoptosis, cell cycle, invasion, and metastasis. In this review article, several studies on NO and biological processes were investigated using PubMed, Scopus, Science direct, and Web of Science. NO acts like a double-edged sword in physiology and pathology of the biological systems. Due to the important role of NO in biological systems, it can be used as a therapeutic goal in various diseases. The aim of this review article was to evaluate the importance and role of NO in biological systems and related process including inflammation, blood clotting, cancer, and metastasis.