Antimicrobial peptides (AMPs) are small peptides which exhibit broad-spectrum antimicrobial activity and often activity against virus. In addition, AMPs exert several functions including endotoxin neutralization, chemotactic and immunomodulating activities, angiogenesis, and wound repair. In this book, the authors discuss the innate immune system and lipopolysaccharides; the biological role of fish antimicrobial peptides; casein-derived peptides; antimicrobial peptides in intestinal inflammation and infection; insect antimicrobial peptides in immunity and mechanisms of action; potential of AMPs as therapeutic tools against infectious disease; and how LC-MS/MS-based quantitative protein profiling can aid mechanistic studies of antimicrobial peptides.