Carried over losses can be applied against future _earnings_ though so if you make $1M and not pay any taxes on it at that point because it's being offset by your past losses. I used it to offset my gains in taxes from selling my company stock (ISOs) when I sold a rental property at a substantial loss shortly after the Great Recession. Had my taxes double checked with an accountant to make sure I did the math right and I was right to the cent.