Anticancer activities of natural antimicrobial peptides from animals. Antimicrobial peptides (AMPs) are natural bioactive peptides found in various organisms, including bacteria, fungi, plants, and animals, and serve as the first line of defense against pathogens. Some AMPs have been proven to possess anticancer activities, defined as anticancer peptides (ACPs). These peptides have the advantages of being able to specifically target cancer cells and have less toxicity to normal tissues. Recent studies have found that marine and terrestrial animals contain a large amount of ACPs. This article introduces animal-derived AMPs with anti-cancer activity, summarizes the types of tumor cells inhibited by ACPs, the mechanisms by which they exert anti-tumor effects, and clinical applications of ACPs. AMPs are natural bioactive peptides with diverse structural properties that are produced by bacteria, fungi, plants, and animals. They are cationic and amphipathic, with a net charge at neutral pH ranging from +2 to +9. Most AMPs are short in length, usually containing 10–50 L-amino acids, which are rich in lysine, arginine, and large amounts of other hydrophobic residues. AMPs can inhibit or kill bacteria by generating electrostatic interactions with negative charges on the surface of bacteria and interacting with various hydrophilic and hydrophobic components. Many AMPs also demonstrate antitumor activity, either by destroying cell membranes or through non-membrane disruptive mechanisms. ACPs have many unique advantages compared to chemotherapy drugs, such as biocompatibility, efficient therapeutic efficacy, low risk of drug resistance appearing in tumor cells, and limited or no toxicity against mammalian cells. ACPs have immunogenicity and low difficulty in synthesis and modification, with a short half-life in vivo, making them possible for clinical anti-cancer drug candidates. AMPs can be classified into α-helical, β-sheet, mixed α-helical/β-sheet, cyclic, and unstructured AMPs based on their secondary structure. The α-helical peptides are the most studied type of AMPs. β-sheet AMPs are typically molecules composed of at least two antiparallel β-sheets, which contain 6 to 8 cysteine residues and are further stabilized by forming two or more disulfide bonds. The third type, α-helix/β-sheet mixed structure, is another major structural motif of some AMPs. The fourth type of cyclic peptides contains one or two disulfide bonds to form this conformation. The unstructured AMPs rich in specific amino acids such as tryptophan, proline, and arginine, and typically around 15 amino acids, exhibit a linear structure. The mechanisms of ACPs underlying their anti-cancer effects include membrane disruptive mechanisms, non-membrane disruptive mechanisms, induction of tumor apoptosis, regulation of tumor necrosis, suppression of tumor angiogenesisAnticancer activities of natural antimicrobial peptides from animals. Antimicrobial peptides (AMPs) are natural bioactive peptides found in various organisms, including bacteria, fungi, plants, and animals, and serve as the first line of defense against pathogens. Some AMPs have been proven to possess anticancer activities, defined as anticancer peptides (ACPs). These peptides have the advantages of being able to specifically target cancer cells and have less toxicity to normal tissues. Recent studies have found that marine and terrestrial animals contain a large amount of ACPs. This article introduces animal-derived AMPs with anti-cancer activity, summarizes the types of tumor cells inhibited by ACPs, the mechanisms by which they exert anti-tumor effects, and clinical applications of ACPs. AMPs are natural bioactive peptides with diverse structural properties that are produced by bacteria, fungi, plants, and animals. They are cationic and amphipathic, with a net charge at neutral pH ranging from +2 to +9. Most AMPs are short in length, usually containing 10–50 L-amino acids, which are rich in lysine, arginine, and large amounts of other hydrophobic residues. AMPs can inhibit or kill bacteria by generating electrostatic interactions with negative charges on the surface of bacteria and interacting with various hydrophilic and hydrophobic components. Many AMPs also demonstrate antitumor activity, either by destroying cell membranes or through non-membrane disruptive mechanisms. ACPs have many unique advantages compared to chemotherapy drugs, such as biocompatibility, efficient therapeutic efficacy, low risk of drug resistance appearing in tumor cells, and limited or no toxicity against mammalian cells. ACPs have immunogenicity and low difficulty in synthesis and modification, with a short half-life in vivo, making them possible for clinical anti-cancer drug candidates. AMPs can be classified into α-helical, β-sheet, mixed α-helical/β-sheet, cyclic, and unstructured AMPs based on their secondary structure. The α-helical peptides are the most studied type of AMPs. β-sheet AMPs are typically molecules composed of at least two antiparallel β-sheets, which contain 6 to 8 cysteine residues and are further stabilized by forming two or more disulfide bonds. The third type, α-helix/β-sheet mixed structure, is another major structural motif of some AMPs. The fourth type of cyclic peptides contains one or two disulfide bonds to form this conformation. The unstructured AMPs rich in specific amino acids such as tryptophan, proline, and arginine, and typically around 15 amino acids, exhibit a linear structure. The mechanisms of ACPs underlying their anti-cancer effects include membrane disruptive mechanisms, non-membrane disruptive mechanisms, induction of tumor apoptosis, regulation of tumor necrosis, suppression of tumor angiogenesis