Organic semiconductors are increasingly important in our daily lives. Hence, gaining insights into their structure–function relationship is crucial to rationally design new materials. This is a multidisciplinary effort. Here, we present four examples of how applying time-resolved EPR (TREPR) spectroscopy to short-lived light-induced triplet states can contribute to our understanding of organic semiconductors: (i) overall orientation and degree of ordering of polymer films on substrates, (ii) detailed electronic structure of a polymer by investigating a series of building blocks with different lengths, (iii) the direct optical excitation of triplet states from the singlet ground state, completing our picture of potential triplet pathways, and (iv) identifying the building block resembling the electronic structure of the corresponding polymer. Taken together, TREPR spectroscopy of triplet states provides highly relevant insight complementary to other methods, adding to our understanding of the structure–function relationship of organic semiconductors.