How will the after market affect the open of the market tomorrow?
In general a stock can open at absolutely any price with no regard for the closing price or after hours price the previous day. The opening price will be determined by the best bid and offer made by people who decide to trade the next day.  Some of the those people may have put orders in on a prior day that are still on the books and matter, but there's a lot of time overnight for people to cancel orders and enter new ones, which is especially likely to happen if there was substantive news overnight. As for what you can do in your case, you have the same options that you always had:  Sell or hold.  If you're selling, you can sell after hours, in the pre-open hours, or during the trading day.  There's nothing we can say about this case that's really any different than we can say about any other stock on any other day.
"No, a jump in market capitalization does not equal the amount that has been invested.  Market cap is simply the stock price times the total number of shares. This represents a theoretical value of the company. I say ""theoretical"" because the company might not be able to be sold for that at all.  The quoted stock price is simply what the last buyer and seller of stock agreed upon for the price of their trade. They really only represent themselves; other investors may decide that the stock is worth more or less than that.  The stock price can move on very little volume. In this case, Amazon had released a very good earnings report after the bell yesterday, and the price jumped in after hours trading. The stock price is up, but that simply means that the few shares traded overnight sold for much higher than the closing price yesterday.  After the market opens today and many more shares are traded, we'll get a better idea what large numbers of investors feel about the price. But no matter what the price does, the change in market cap does not equal the amount of new money being invested in the company. Market cap is the price of the most recent trades extrapolated out across all the shares." the financial information is generally filed via SEDAR (Canada) or SEC (US) before the conference call with the investment community.  This can take before either before the market opens or after the market closes.  The information is generally distribute to the various newswire service and company website at the same time the filing is made with SEDAR/SEC. You miss the step where the return being doubled is daily. Consider you invested $100 today, went up 10%, and tomorrow you went down 10%. Third day market went up 1.01% and without leverage - got even. Here's the calculation for you: day - start - end 1      $100    $120  -  +10% doubled 2      $120    $96   -  -10% doubled 3      $96     $97.94 - +1.01% doubled So in fact you're in $2.06 loss, while without leveraging you would break even. That means that if the trend is generally positive, but volatile - you'll end up barely breaking even while the non-leveraged investment would make profits. That's what the quote means. edit to summarize the long and fruitless discussion in the comments: The reason that the leveraged ETF's are very good for day-trading is exactly the same reason why they are bad for continuous investment. You should buy them when there's a reasonable expectation for the market to immediately go in the direction you expect. If for whatever reason you believe the markets will plunge, or soar, tomorrow - you should buy a leveraged ETF, ride the plunge, and sell it in the end of the day. But you asked the question about volatile markets, not markets going in one direction. There - you lose. What you choose to invest in depends largely on your own goals and time horizon.  You state that your time horizon is a few decades. Most studies have shown that the equity market as a whole has outperformed most other asset types (except perhaps property in some cases) over the long term.  The reason that time horizon is important is that equities are quite volatile. Who knows whether your value will halve in the next year? But we hope that over the longer term, things come out in the wash, and tomorrow's market crash will recover, etc.  However, you must realize that if your goals change, and you suddenly need your money after 2 years, it might be worth less in two years than you expect. "If there are no limit orders on the opposite side of the book when your market order gets its turn for execution, it should be rejected by the market.  A market order should generally not ""sit on the book"" like your question suggests waiting for another order to arrive.  Thus, the situation that you describe should not happen in an ordinary market that is operating in an orderly fashion. This is not to say that your order cannot ""sit"" for a while in a queue - If there is heavy volume, orders will be executed in order, so your your market order may have to wait for orders entered ahead of it to be processed.  But once its turn comes up, that's it. There are some related points to consider: I should caution that my answer is biased a bit to US stock markets, whereas you asked about currency markets.  I believe the same basic principles apply, but I'd be swayed by someone with evidence to the contrary.  I'd also note that currency tends to be more liquid than stock, so I think it's less likely that this situation would come up.  Maybe possible for a ""weak"" currency or a currency that experiences a sudden crisis of some sort." Usually backtests for (long-term) strategies are evaluated on a end-of-day basis where you only consider close prices. If your strategy performs well in these backtests, hopes are that if you use a market-on-close (MOC) order your performance will not diverge too much from the backtest. The fact that it won't diverge much is important if you keep backtesting the strategy along with the real trading to see regime changes or similar.  If you used end-of-day prices for the backtests but some arbitrary intraday market order, you'd have some difficulties to explain deviations between the two. What it is:  MOC orders can be submitted during the day, but they won't be executed until shortly before the market (or more precise the current session) closes. "As someone that works in and with the ag and commodity markets, my understanding, is limited but I hope it helps.   Aside from needing an infrastructure built to maintain the vast amounts of data, communication, pricing, banking information, having a closing time for our markets helps to ensure value and fair trade. Capitalism is centralized around creating a fair value for ventures to sell, where prospective buyers opt to purchase or invest. For example, where I attended university, there were many coffee shops close to campus, but only one stayed open all night, and as such had operational cost much higher than other shops, forcing them to charge more for a cup of coffee. While this example is crude, the idea is the same. By maintaining""bank"" hours, product or strategies and developments meant to increase projected value are allowed to occur.   I imagine that if the markets did not close, my coffee prices would skyrocket for an increased demand, that of which the supply chsin, and farmers, would struggle to meet. The same would be said of grain, salt, oils, etcetera." I don't have all the answers. On a illiquid stock, such situations do arise and there are specific mechanisms used by exchanges to match the order. It is generally not advisable to use market order on illiquid stock. There are lots of different variations here. I guess this comes down to specifications for individual exchanges, but I'm wondering if there's a standard here or a way to approach it from basic rules that clears up all these situations. There are quite a few variations and different treatments. Market order that are placed when the market is closed or just around market opening are traded at Market Open price that each exchange has a formulae to calculate. In the process Market Buy are matched to Market Sell at the Exchange calculated price. Not all order get matched and there could be spill over's. These are then matched to limit orders. Is this determined based on which sell order came first, or based on which would result in the best deal for the incoming buyer? Generally Market orders have highest priority of execution.