<prompt> Human: One of the common things I read on this sub and similar websites is the importance of choosing very low expense ratios, because that money really adds up over time as your portfolio grows.Many like to point to Vanguards often low expense ratios and say thanks to these low fees, I estimate 120k in additional savings when Im 60! My question is ifwhy do these actually matter? Arent they built in to the trading price of the fund? In other words, lets say company A has a find with an expense ratio of 0.15 and company B has 0.85, yet the two funds price histories over the same time are charted and identical. It seems to me that there is no difference in value gainedlost to my portfolio by choosing one over the other.Am I missing something fundamental here? Can someone explaingive an example of why it matters? Im fairly young and new to investing so Im trying to soak up as much info as possible. Thanks!  <response>: In other words, lets say company A has a find with an expense ratio of 0.15 and company B has 0.85, yet the two funds price histories over the same time are charted and identical.You cant go back in time and invest, though. Theres no reason to think B is going to outpeform A for the next 30 years. Its actually the opposite: company B is more expensive, so since youre guaranteed to spend more money in fees for every dollar earned, the odds are over the long run, that you have a smaller net return with company B.In other words, past performance doesnt predict future results. Which is obvious, because were not all clairvoyant and know which funds will do the best tomorrow, the next day, and the next, etc. If the past predicted the future, every investor in the world would just pile all their money into one company say, whatever company did best today since thats the one guaranteed to do better than all others.But anyway, it matters. For example, with an 8 return for 25 years and 10kyr invested, the 0.15 ER saves you 79,000 This is with otherwise identical funds. 