The energy transfer upconversion (ETU) mechanism is known to be the most efficient route for the conversion of near infrared (NIR) light to visible emission in Ln3+-co-doped systems. In this work, we examined these energy transfer (ET) processes in Yb3+,Er3+-co-doped fluoride single crystals. Because of their low phonon energy, high thermal dissipation, chemical stability and high transmission in the ultraviolet, visible, and NIR, these materials are ideal systems to study such processes. Here, we focus on the influence of the concentration of the sensitizer Yb3+ on the optical and upconversion properties of three different fluoride hosts doped with Yb3+ and Er3+ as a function of excitation power density and compare direct Ln3+-excitation and excitation in the NIR via ET.