Le Chatelier's principle states that a system at equilibrium responds to a stress by shifting to partially counteract it. Concentration changes: adding reactant shifts right, adding product shifts left, removing product shifts right. Pressure changes (gases only): increasing pressure shifts toward fewer moles of gas. For N2 + 3H2 <=> 2NH3 (4 moles -> 2 moles), high pressure favours NH3. Adding inert gas at constant volume does NOT shift equilibrium (partial pressures unchanged). Adding inert gas at constant pressure shifts toward MORE moles of gas (volume increases, partial pressures decrease). Temperature: increasing T shifts toward the endothermic direction. This is the ONLY change that alters K. A catalyst affects neither equilibrium position nor K — it only speeds attainment.