Metal halide perovskite semiconductors possess excellent optoelectronic properties, allowing them to reach high solar cell performances. They have tunable bandgaps and can be rapidly and cheaply deposited from low-cost precursors, making them ideal candidate materials for tandem solar cells, either by using perovskites as the wide-bandgap top cell paired with low-bandgap silicon or copper indium diselenide bottom cells or by using both wide- and small-bandgap perovskite semiconductors to make all-perovskite tandem solar cells. This Review highlights the unique potential of perovskite tandem solar cells to reach solar-to-electricity conversion efficiencies far above those of single-junction solar cells at low costs. We discuss the recent developments in perovskite-based tandem fabrication, and detail directions for future research to take this technology beyond the proof-of-concept stage.