The rapid advancement of thin-film photovoltaic (PV) technology increases the real possibility of large-area Si-based tandems reaching 30% efficiency, although light in these devices must be managed carefully. We identify the optical requirements to reach high efficiencies. Strict conditions are placed on material parasitic absorption and transmission of contacts: Absorption of 20% of sub-bandgap light leads to the required top-cell efficiencies of 18% at a bandgap of 1.5 eV to break even and 23% to reach tandem efficiencies of 30%. Perovskite-silicon tandem cells present the first low-cost devices capable of improving standalone 25% efficiencies and we quantify the efficiency gains and reduced thickness afforded by wavelength-selective light trapping. An analytical formalism for Lambertian tandem light trapping is introduced, yielding stringent requirements for wavelength selectivity. Applying these principles to a perovskite-based top cell characterized by strong absorption and high luminescence efficiency we show that tandem efficiencies greater than 30% are possible with a bandgap of E g = 1.55 eV and carrier diffusion lengths less than 100 nm. At an optimal top-cell bandgap of 1.7 eV, with diffusion lengths of current vapor-deposited CH 3 NH 3 PbI x Cl 1-x perovskites, we show that tandem efficiencies beyond 35% are achievable with careful light management.