The most common types of peptides consist of signal peptides, carrier peptides, enzyme-inhibitor peptides, and neurotransmitter-inhibitor peptides, which all have specific functions. While you’ll often find "peptides" placed front and center on the actual product packaging, ingredient lists often categorize peptides by how many amino acids a peptide contains, such as dipeptide (two), tripeptide (three), and hexapeptide (six). For our list of the best peptide serums, we considered each product’s performance across five primary categories: product ingredients and efficacy, packaging, fragrance, texture, and product wear.