Timing closure is the iterative design process ensuring all signals meet setup and hold timing requirements, where slack (timing margin) indicates whether paths meet constraints and the critical path (slowest path) determines maximum operating frequency. Timing fundamentals: setup time (data must arrive before clock edge), hold time (data must remain stable after clock edge), and slack (margin beyond requirement—positive is good, negative violates timing). Critical path: the path with worst (most negative or least positive) slack, limiting chip frequency. Timing closure flow: synthesis generates initial netlist, timing analysis identifies violations, optimization techniques (gate sizing, buffering, logic restructuring) fix violations, placement optimization reduces wire delay, and clock tree synthesis balances clock arrival. Iterations continue until all paths meet timing. Challenges: process variation (fast/slow corners), voltage/temperature effects, and clock domain crossings. Tools: static timing analysis (STA) checks all paths without simulation. Sign-off corners: analyze at multiple PVT (process, voltage, temperature) combinations. Timing closure often dominates design schedule at advanced nodes as shrinking margins amplify sensitivity. Design techniques: pipelining (reducing combinational depth), retiming (moving registers), and microarchitecture changes for critical paths.