The monolithic integration of high-performance III-V compound semiconductor materials and devices with low-cost, high-availability Si substrates remains a ‘holy grail’ of photovoltaics materials research, despite decades of effort. However, beyond merely a low-cost materials platform, III-V/Si multijunction solar cells utilizing active Si sub-cells—a “Si-plus” architecture—also hold the potential for high conversion efficiencies on par with pure III-V multijunction structures. One of the most promising approaches to this end is via direct heteroepitaxial GaP/Si integration and bandgap/lattice constant engineering via compositionally-graded GaAsyP1-y and/or Ga1-xInxP alloys to reach III-V compositions that possess bandgaps ideally matched to the underlying Si. Substantial work by an international collaborative team led by The Ohio State University has, in recent years, delivered the first demonstrations of monolithic epitaxial GaAsP/Si dual-junction (2J) and GaInP/GaAsP/Si triple-junction (3J) solar cells. Continued effort toward optimization the 2J prototypes has brought ARC-projected efficiencies up to the point of competitions with conventional Si cells, with clear pathways for further improvement. This presentation will discuss recent progress in both epitaxial materials/process development and device development toward the production of high-performance GaAsP/Si 2J solar cells. Topics will include optimization and characterization of the GaP/Si integration process and GaAsyP1-y metamorphic grading, advanced Si bottom cell and GaAs0.75P0.25 top cell design and development, development and optimization of high-performance metamorphic tunnel junctions, and up-to-date results from fully-integrated tandem devices.