It goes without saying that you'll need your pump of choice, but make sure it's either fully charged or you've packed the charger. Don't forget any of the parts, such as the tubing, connectors, or membranes. You'll also need some compatible bottles that are sterilized and ready to use — the number you need will depend on how long you'll be out and how much milk you tend to produce. You should also pack extra breast pads and some wet wipes or a washcloth to deal with any spills. Optional extras include a snack, a book (or something to entertain yourself), breastmilk storage bags, and a Sharpie for labeling.