Peptides are a promising class in drug discovery that merge the benefits of modularity and synthetic convenience of small molecules with the remarkable selectivity and potency of biologics. However, peptides are also prone to exhibit low cell permeabilities, poor metabolic stabilities, and high flexibilities that inhibit their clinical applications. One of the most powerful strategies to overcome the limitations of therapeutic peptides is macrocyclization. Through macrocyclization, structures are reinforced, peptide conformations are stabilized, and binding efficiencies for biological targets are improved. In recent decades, several macrocyclization procedures have been published. Traditional two-electron transformations such as ring-closing metathesis, azide-alkyne cycloadditions, other transition-metal-catalyzed methods, conjugate additions, nucleophilic aromatic substitutions, and multicomponent reactions have all been utilized for peptide macrocyclizations. Furthermore, recent developments in photoredox chemistry have enabled the use of open-shell intermediates to engage in macrocyclizations through mechanisms that are orthogonal to their two-electron analogues. Despite the progress achieved in the synthesis of macrocyclic peptides, several challenges continue to demand novel macrocyclization conditions, primarily maneuvering the ground-state trans geometries of the acyclic peptide backbone into a proper orientation for macrocyclization and inhibiting oligomerization side-reactions. Therefore, three solid-phase photochemical peptide modifications have been explored. The first procedure is a solid-phase photochemical hydroalkylation that was utilized to install a broad scope of medicinally-relevant substrates onto resin-bound Giese acceptors. This procedure represents the first application of photochemistry to solid-phase peptide synthesis. The second procedure utilized the solid-phase hydroalkylation to cyclize a series of on-resin peptides, including two variants of the oxytocin antagonist Atosiban. The third procedure is currently in progress and utilizes a relatively new photochemical electron-donor-acceptor complex process to develop a novel cyclization mechanism that involves pre-organization of the cyclic precursor by complexation of the reactive termini. The results herein illustrate that on-resin photochemistry can be used to overcome several challenges associated with modifying acyclic peptides and cyclizing macrocyclic precursors.