4-Hydroxyproline (4-HyP), a prevalent amino acid in collagen, plays a critical role in stabilizing the collagen triple helix. While its structural and stabilizing effects have been studied using synthetic triple helices, deeper insights into its conformational preferences and electronic interactions have been gained through modern quantum chemical methods. This chapter focuses on computational studies involving 4-HyP, the prolyl–4-HyP–glycine (PO4G) tripeptide, and collagen triple-helix models. Density functional theory (DFT), widely adopted for its balance between accuracy and computational efficiency, has shown that the (R)-stereoisomer of 4-HyP favors the exo ring pucker and trans peptide bond. Natural bond orbital (NBO) analysis further revealed that these preferences are stabilized by n→π* and σ→σ* charge-transfer interactions, respectively. Recent studies highlight how such stereoelectronic effects, particularly within the abundant PO4G motif, contribute to the overall stability of the collagen triple helix. Given the repetitive tripeptide nature of collagen, periodic DFT methods offer a powerful framework to investigate its structure and stability. This chapter reviews the fundamental principles of DFT and NBO analyses and discusses the electronic origins of charge-transfer interactions in the context of collagen stabilization.