Held items can significantly impact a Pokémon's performance in battle. Choice Band/Scarf/Specs boost one stat but lock you into one move. Life Orb boosts damage at the cost of HP. Leftovers restore HP each turn. Assault Vest boosts Special Defense but prevents status moves. Focus Sash allows a Pokémon to survive a fatal hit with 1 HP. Many other items exist to boost specific types, prevent status, or aid in evolution. Experiment to find what works best for your team.
