Photothermal therapy (PTT) is an attractive method for combating tumours and bacterial infections in which light is used to destructively heat cells. Upon photoexcitation, photothermal materials (PTMs) undergo efficient nonradiative decay, thereby generating heat, which can kill targeted tissues via apoptosis and necrosis pathways. Several materials such as gold nanoparticles, metal-organic frameworks, lanthanide-based nanoparticles, and small organic molecules have been investigated as PTMs to treat diseases both in vitro and in vivo. In contrast to photodynamic therapy (PDT), in which photoexcitation is used to generate cytotoxic singlet oxygen using photosensitizers, PTT does not require the presence of oxygen within tissues and can thus be effective in hypoxic tumour environments. Interestingly, however, PDT and PTT have recently been found to exhibit synergistic effects when deployed together. Such a multimodal strategy is also minimally invasive, since phototheranostic nanomaterials can be targeted to specific tissues. Porphyrins and related molecules are widely used as photosensitizers for PDT, and are particularly desirable because of their strong Q-band absorbance between 600 -800 nm. Tissue components such as blood, melanin, and water do not absorb wavelengths of light between 650 -1400 nm as strongly as visible light, making materials that absorb within this range optimal to avoid damaging healthy tissues. Interestingly, a class of porphyrin analogues termed isoporphyrinoids (after the parent compound isoporphyrin ), in which aromaticity has been interrupted by the saturation of one meso carbon, exhibit further reduced HOMO-LUMO gaps relative to their aromatic counterparts resulting in strong near-infrared (NIR) absorption. (Scheme 1) One such class of molecules is phlorins, which are dihydroporphyrins that were discovered and named by Woodward in connection with his celebrated total synthesis of chlorophyll. Another class, isocorroles, which are formal tautomers of corrole, are of more recent provenance, with several examples reported in the last several decades. Unlike many metalloporphyrins and metallocorroles incorporating 5d metals, which exhibit NIR phosphorescence, transition metal isocorroles and phlorins exhibit weak luminescence, if any, suggesting efficient non-radiative decay pathways. Some 4d and 5d transition metal phlorin and isocorrole materials, however, sensitize singlet oxygen formation with moderate efficiency, consistent with long-lived excited triplet states. Scheme 1. Relationship between isoporphyrinoid materials, and identification of photothermally active candidate materials.