I filter, depending on the trail and location. On the AT I don't filter. On the PCT I started using Aqua Mira, and then stopped after the first couple hundred miles. On the CDT I filtered through New Mexico because of all the water sources used by Cattle. I did not filter on the CT, but I did filter on the AZT for the same reason I did on the CDT. Basically, if cattle consistently use the water sources I filter with a Sawyer. FYI - I got Giardia on my PCT hike, but have never had it since that hike. :-)