In this review, the recent progress of the heavy atom-free triplet photosensitizers was summarized. The general approaches include to attain S1/Tn states sharing similar energy levels, or proper molecular geometry to satisfy the reservation of the angular momentum reservation in intersystem crossing (ISC). ISC via the higher singlet excited state (Sn, n > 1) --> Tm (m > 1) state was also discussed, which is a rarely reported phenomenon. The ISC of some Bodipy dimers was proposed to be via the ‘doubly excited states’, but the recent studies show that the ISC mechanism of these Bodipy dimers is charge separation/recombination. These new progresses in the study of triplet photosensitizers are useful for photovoltaics, photodynamic therapy, photocatalysis, as well as in fundamental photochemistry studies.