This is the tried and true method of killing microscopic beasties, but it takes a lot of fuel. Fires work, but they're not exactly minimum impact, plus your water ends up tasting like woodsmoke. If you do use a fire, decant the water from one bottle to another to get rid of the sour burnt taste. Or throw in a couple of herbal tea-bags. There's a lot of disagreement over just how long you need to boil water for.
Some agencies recommend five minutes or even longer, especially at higher altitudes.
Others say that a strong rolling boil at any altitude will kill the cooties. Iodine is available in liquid, crystals, or tablets. Read the directions for how much to use and how long to wait more iodine and longer waiting times are required in cold or especially dirty water. To mask the flavor, you can drop in tablets made by Potable Aqua. Few pieces of backpacking equipment are as vilified as water filters.