How do you handle voltage domain crossing in VLSI?

Voltage domain crossing occurs when signals pass between regions at different supply voltages. Level shifters convert voltage levels: Low-to-high shifter boosts voltage, High-to-low shifter reduces voltage (simpler). Isolation cells prevent floating or illegal values when power domain is off. Implementation: Insert level shifters at domain boundaries, use isolation for power-gated domains, consider timing impact of shifters, and verify correct shifter insertion. Special cells handle retention signals. UPF (Unified Power Format) specifies power intent for automated tool handling.