The chapter discusses the role and mechanisms of antimicrobial peptides in host defense. These peptides, produced by various organisms, are part of the innate immune system and play a crucial role in defending against microbial infections. They are typically short, positively charged, and amphipathic, with diverse secondary structures including α-helices, β-sheets, loops, and extended forms. Antimicrobial peptides can be constitutively expressed or induced in response to pathogens, and they exhibit potent antimicrobial activities against bacteria, fungi, and some viruses. They also have immunomodulatory functions, enhancing phagocytosis, prostaglandin release, and immune cell recruitment. The chapter highlights the structural requirements and modes of action of these peptides, emphasizing their ability to interact with viral and bacterial targets, such as heparan sulfate, viral envelopes, and bacterial membranes. Additionally, it explores the antiviral and antibacterial activities of antimicrobial peptides, detailing how they can block viral entry, interfere with viral replication, and permeabilize bacterial membranes. The chapter also discusses the importance of peptide structure and charge in determining their antimicrobial efficacy and the potential for developing antimicrobial peptides as therapeutic agents.The chapter discusses the role and mechanisms of antimicrobial peptides in host defense. These peptides, produced by various organisms, are part of the innate immune system and play a crucial role in defending against microbial infections. They are typically short, positively charged, and amphipathic, with diverse secondary structures including α-helices, β-sheets, loops, and extended forms. Antimicrobial peptides can be constitutively expressed or induced in response to pathogens, and they exhibit potent antimicrobial activities against bacteria, fungi, and some viruses. They also have immunomodulatory functions, enhancing phagocytosis, prostaglandin release, and immune cell recruitment. The chapter highlights the structural requirements and modes of action of these peptides, emphasizing their ability to interact with viral and bacterial targets, such as heparan sulfate, viral envelopes, and bacterial membranes. Additionally, it explores the antiviral and antibacterial activities of antimicrobial peptides, detailing how they can block viral entry, interfere with viral replication, and permeabilize bacterial membranes. The chapter also discusses the importance of peptide structure and charge in determining their antimicrobial efficacy and the potential for developing antimicrobial peptides as therapeutic agents.