If you use a powdered formula, we would take the bottle with the proper amount of water inside. I did not refrigerate, just kept in diaper bag. You can warm on stroller, as a pp suggested. I would place the proper amount of formula powder in a formula keeper in room for multiple feedings during the day & fill that many bottles of water as well. Some formula are for sale in packets, but they only make 4 oz or multiples thereof, so not always convenient. When ds was ready to eat, we would mix the formula & the premeasured water in the bottle by shaking &(I also carried a small whisk that would fit in bottle in case of clumping) he had a warm bottle. I also didn't have to worry about keeping formula cold enough, getting warm enough to eat, or how long bottle was mixed prior to eating. I filled my bottles & formula container the night before so everything was ready to go in the morning. I hope this wasn't too confusing: it worked for us when ds was taking 6oz at a time so I didn't have to waste or guess at 1/2 a packet.