The World Health Organization has a pdf explaining how to safely prepare formula for infants. I was NOT aware of these guidelines when we offered our first baby a bottle of formula (which he soundly rejected), and thankfully I was able to breastfeed our first two babies without problems. Our third was our micro-preemie and in order to get him enough calories I needed to pump then add a special preemie formula to the breastmilk. I wish I had been given more explicit instructions about how to ensure we were doing it safely! I hope the above information will help other parents. The most crucial points to remember are keeping things clean, boiling the water, and then cooling it quickly (in the refrigerator) to a safe drinking temperature so it's not in the "danger zone" for bacterial growth for long. You also need to toss formula left at room temperature for too long.