Twenty eight years have passed since a pioneering LEED study of Ge(110) surface reconstruction was published by Olshanetsky et al. in 1977, and now natures of the reconstruction are rapidly being clarified. In the present article, experimental studies on the structures of the reconstructed surfaces so far performed are surveyed, and a 16×2 structure is concluded to be only a stable structure on reconstructed Ge(110) surfaces at temperatures below 430^oC. Atomic geometries of the 16×2 and a metastable c(8×10) structures are studied through STM observations and first-principle structural optimization calculations, and new models of the two structures are presented, in which five-membered clusters of Ge adatoms are shown to be fundamental structural elements of their structures.