It was only two decades ago that the reaction to the idea of measuring shape distortions of galaxies by gravitation lensing for reliably constraining cosmological parameters turned from skepticism to enthusiasm. Today, the technique called weak-lensing is routinely used to study dark matter distribution in galaxy clusters, average mass profiles of galaxies, and the large scale structures in the universe. The next decade will be the most exciting era when we to collect accurate cosmic shear data from billions of galaxy shapes. I will provide detailed introduction to gravitational lensing while highlighting some of the key milestones in the field. Ambitious future multi-billion dollar projects such as LSST, EUCLID, and WFIRST will be discussed. I will emphasize that in order to achieve their proclaimed scientific goals, substantial amounts of concerted efforts are required to overcome systematics. Finally, I will present my most recent results obtained from colliding galaxy clusters, which are often dubbed as "cosmic particle accelerators".