Solar cell efficiency is extremely leveraging for reducing the levelized cost of energy (LCOE) in CPV systems. Over the past three years, Spectrolab has developed and introduced four generations of III-V multi-junction solar cells for terrestrial concentrator applications, with continuous improvement in the conversion efficiency from 37% for C1MJ to 40% for C4MJ. This continuous increase in conversion efficiency has been achieved by improvements in the cell design and quality of epitaxial layers, as well as by improvements in wafer fabrication processes. Design and fabrication of metal gridlines have large impact on the solar cell performance. Optimization of the gridline architecture is extremely important to achieve high efficiency. Modeling studies were carried out to find optimum gridline parameters (viz. width, thickness and spacing) as a function of cell size and level of light concentration. Wafer fabrication processes were continuously modified to increase the aspect ratio of the gridlines, which is critical to minimize the metal obscuration while simultaneously maintaining the high electrical conductance of the grid structure. The results of modeling studies are compared with experimental results for various gridline designs. These improvements in the gridline design have yielded ∼1.2% absolute increase in solar cell efficiency between C1MJ and C4MJ cell generations. Another model was developed to estimate the effects of non-normal angles of light incidence on cell performance, that included the effects of gridline shadowing and light reflection from the gridline sidewalls. In addition, new all-chemical mesa etch processes have been developed for cell isolation, the results of which will be presented and implications from the wafer fabrication as well as cell performance perspectives will be discussed.