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A share, like most other things, is worth what someone else will pay you for it. That depends on the psychology of crowds, greed, fear and momentum and all kinds of exciting things. In terms of what should determine that price, however, two factors must logically override all others: the earnings stream that a company will generate for you in the future, and the interest rate at which those earnings should be discounted. That makes this a classically difficult juncture for market valuation. The US and world economies are growing faster than had been thought, which makes it fair to expect higher earnings that need to be discounted at a higher rate; after all those calculations have been made, does that mean share prices should go up or down?