The most success in terms of upconversion (UC) efficiencies has been obtained in the visible region. In particular, red-to-blue TTA-UC have been reported with UC quantum yields (FUC) as high as 42% (out of a theoretical maximum of 50% owing to the two-to-one nature of the UC process), while other spectral regions have proven more challenging. Upconverting near-infrared or infrared light to the visible region, which is especially important for biological and photovoltaic applications, have seen much lower efficiencies with a FUC of 8% at best. Similarly, the performance of visible-to-ultraviolet (vis-to-UV) TTA-UC systems suffer from limited efficiencies. Significant progress has however been made recently, with reports on FUC of around 10% for three different systems. Still, there is no fundamental reason as to why much higher efficiencies would not be possible.