Lactoferricin (LFcin) was initially identified as an antimicrobial peptide derived by pepsin digestion of lactoferrin (LF), a multifunctional innate-defense protein in milk. Various synthetic analogs of LFcin have also been reported. LFcin inhibits a diverse range of microorganisms such as gram-negative bacteria, gram-positive bacteria, yeast, filamentous fungi, and parasitic protozoa, including some antibiotic-resistant pathogens. LFcin kills target organisms by membrane perturbation and acts synergistically with some antimicrobial agents. LFcin exhibits numerous biological activities in common with those of LF. Whereas LFcin suppresses the activation of innate immunity by microbial components such as lipopolysaccharide (LPS) and CpG DNA, the peptide itself activates immunity. Administration of LFcin analogs has been shown to protect the host via direct antimicrobial activity and immunostimulatory effects in several infection models of mice. Here we present a comprehensive review of investigations of LFcin and related peptides.