This paper presents the design, implementation, and experimental results of a highly efficient concurrent dual-band GaN-HEMT power amplifier at 1.8 GHz and 2.4 GHz. A bare-die approach, in conjunction with a harmonic source-pull/load-pull simulation approach, are used in order to design and implement the harmonically tuned dual-band PA. For a continuous wave output power of 42.3 dBm the measured gain is 12 dB in the two frequency bands; while the power added efficiency is 64% in both bands. Linearized modulated measurements, using concurrently 10MHz LTE and WiMAX signals, show an average PAE of 25% and and adjacent channel leakage ratio of -48 dBc and -47 dBc at 1.8 GHz and 2.4 GHz, respectively. \ua9 2012 IEEE