Galaxy clusters are the largest gravitationally bound objects and the weak lensing technique is a powerful tool to determine their masses without any dynamical assumption. I would like to introduce two main cluster studies I am involved with. One project (MC^2: Merging Cluster Collaboration, http://www.mergingclustercollaboration.org) is focused on merging clusters as cosmological experiments to test the dark matter properties. The masses of sub-clusters estimated by weak lensing are an important initial condition for cluster merger simulations with which we can test different kinds of dark matter. I will introduce my current study on Abell 521 regarding this topic. The other project (SeeChange) targeted SZ clusters for testing the standard cosmological model. Because massive clusters can be rarely found at high redshift, finding too many massive clusters in high-z can support departure from the standard cosmological model. A recent paper on the most massive SPT cluster at z >1, SPT2106 (https://arxiv.org/abs/1910.04775) will be presented.