Abstract　　　
Massive galaxy clusters can act as gravitational "cosmic telescopes", considerably magnifying both the apparent luminosity and size of background sources. Gravitational lensing by massive galaxy clusters has been highlighted as a powerful tool in the discovery and study of the properties of faint high-redshift galaxies that otherwise would have remained undetected. In this talk, I will show recent discoveries of high-redshift galaxies utilizing 16-band HST observations of lensing clusters obtained as part of the CLASH Multi-Cycle Treasury program. Then I will talk about our ongoing works on selecting and analyzing Lyman break galaxies at z~1-3 with the CLASH data. Understanding these LBGs is important because they are at redshifts corresponding to the peak epoch of the global star formation rate, and has significant implications on the higher-redshift LBGs because of the similar dropout selection.