The immune system is a host defense system that protects against diseases mostly by eliminating aberrant cells, for instance pathogen-infected or tumorigenic cells. In this process, immune killer cells, namely cytotoxic T lymphocytes (CTLs) from adaptive immune system and natural killer (NK) cells from innate immune system, play a key role. To be able to exert their killing function, CTLs and NK cells have to be properly mobile to search for their targets.  Once the target cells are identified, CTLs or NK cells will form an intimate contact with the target cells, which is termed immunological synapse (IS). Subsequently, the killing machinery will be oriented toward the IS to deliver the lethal hits. CTLs and NK cells use similar killing machineries to destruct their target cells: lytic granules and Fas/FasL pathway. Lytic granules contain cytotoxic proteins including perforin and granzymes, which will be released into the cleft at the IS to induce the death of target cells. Fas ligand (FasL) on killer cells can bind the Fas receptors on the surface of target cells to initiate the apoptosis of the latter. The killing processes are regulated by intracellular and extracellular factors. For intracellular factors, we have identified that 1) actin-binding protein profilin-1 is a negative regulatory molecule for T cell migration, and 2) tethering to certain recycling endosomes is essential for lytic granules to be released at the IS. In terms of extracellular factors, our results show that 1) killing efficiency is enhanced by the presence of non-target bystander cells; 2) effector T cells substantially boost killing activity of NK cells; and 3) high glucose can enhance killing efficiency of CTLs.