Highly efficient photon upconversion in solution is demonstrated for the first time. Transparent colloidal solutions of Yb/Er and Yb/Tm doped NaYF4 nanocrystals show bright upconversion emission in the red, green, and blue spectral region by excitation around 1 mum with a laser of moderate power (see Figure and also inside front cover). These new systems show a substantial improvement on the order of 10(8) compared to previously reported systems.