Indium gallium nitride is being widely investigated for next generation ultra-high efficiency multi-junction (MJ) photovoltaic (PV) solar cells due to the materialís potentially suitable absorption match to the solar spectrum [1, 2]. However, InGaN solar cells will require tunnel junction interconnects between cells that can achieve peak tunnel current densities of the order of 10-100A/cm2, in order to avoid current limiting in a monolithic configuration, especially under high solar concentration (up to x1000). In this presentation we have examined the theoretical tunneling behaviour of InxGa1-xN:InyGa1-yN p++:n++ junctions for both the homojunction (x=y) and heterojunction (x<>y) cases, assuming growth on a c-plane template. Using a commercial device simulator tool and taking into account the polarisation fields where appropriate, we have examined the range of tunneling peak current densities achievable as a function of In composition and doping level in the tunnel junction layers. The optimum tunnel junction design configuration for a recently proposed  35% efficiency 3-junction cell on Si comprising of InGaN layers with bandgaps of 2.0 and 1.5eV for the top and middle cells, respectively, is presented. The implications for current- and lattice matching between cells are also discussed. References: 1. Wu, J., et al., Superior radiation resistance of In1-xGaxN alloys: Full-solarspectrum photovoltaic material system. Journal of Applied Physics, 94(2003)6477. 2. Horie, M., et al., MOVPE growth and Mg doping of InxGa1-xN (x similar to 0.4) for solar cell. Solar Energy Materials and Solar Cells, 93(2009)1013. 3. Simon, J., et al., Polarization-Induced Zener Tunnel Junctions in Wide-Band- Gap Heterostructures. Physical Review Letters, 103(2009)026801. 4. Hsu, L. and W. Walukiewicz, Modeling of InGaN/Si tandem solar cells. Journal of Applied Physics, 104(2008)024507.