Organic upconversion is a unique process in which low-energy light (usually NIR light) is converted to high-energy light through either two-photon absorption (TPA) mechanism or triplet-triplet annihilation (TTA) mechanism. Both TPA upconversion (TPA-UC) and TTA upconversion (TTA-UC) have been actively investigated in recent years due to their showing many intriguing applications in optics, biophotonics, and solar energy utilization. Although they show somewhat similarity (i.e., belonging to nonlinear two-quantum process and needing focusing excitation light), TPA-UC and TTA-UC are very different, such as in mechanism, molecular design, characteristics involved and potential applications. Here, we collectively reviewed these two kinds of upconversion and compare their respective characteristic and merits. We also present recent advances made in the areas of TPA- and TTA-UC, the remaining challenges and opportunities, with a particular emphasis on molecular engineering of these two active upconversion materials.