UNG does not decay. I think there is a lot of confusion here on this thread. Any ETF that replicates a futures product will suffer from contango or benefit from backwardation in the futures roll. You can synthetically replicate this on your own with futures. In the case of natural gas there is a contango which you means you can sell nat gas in a deferred month that is higher then the current price is today. If nat gas goes lower in price, you make money. Then when it's time to roll you get to re-sell nat gas at a higher price and keep repeating the process. You don't need the ETF to do that. Same with USO. It's not a function of decay. You are benefiting from the contango.