In this paper, we introduce a study for the performance of intensity modulation (IM) techniques over gamma-gamma (GG) distributed free-space optical (FSO) channels. We derived closed form expressions for the average bit-error rate (BER) of thermal noise-limited FSO systems adopting various IM techniques. The effects of the atmospheric turbulence and the propagation wavelength are studied. A comparison among different IM techniques in term of transmitted peak power to achieve certain BER is presented. Results show that OOK has the best BER compared with other IM techniques. In addition, according to peak power, PPM and MPPM are more power efficient than DPPM and DMPPM, respectively.