The proteins and nucleic acids that are the targets of most prescribed drugs can be classified according to their structure and mechanism of action at the molecular level. In this course, basic concepts of enzyme action such as the mechanisms of enzyme catalysis, the Michaelis-Menten and pre-equilibrium equations, steady-state approximations, allostery and cooperativity will first be covered. Major classes of therapeutic targets will then be discussed with an emphasis on their normal biochemical roles that are exploited for therapeutic intervention. The mechanisms of action of drugs acting on enzymes (antiviral and antimicrobial agents) on nucleic acids and on the cytoskeleton (anti-cancer agents) will be of special interest. The concept of rational cancer therapy will also be covered with examples of drugs targeting growth factors signalling pathways that are dysregulated in cancers.