Will a stop order get triggered if the floor is hit and trading is halted?
During a circuit breaker, no trading occurs. These policies have been implemented to maintain exchange liquidity since the NYSE nearly went bankrupt during the 1987 crash because many members had become insolvent. If an order is filled before the halt, it will stand unless busted.  During the Flash Crash, many orders were busted. quantycuenta is right, if a halt is in place, then no trading will occur, simple as that. But in the practice of risk management it is a little different. Want to remind you that you are assuming that trading is halted immediately upon the drop in price. That doesn't always happen, so if there is any time between the actual price drop and halt of trading, then it is possible that your order will be filled, depending on how liquid your security is. Also not every security has circuit breakers in place and the exact requirements to trigger a breaker is not public information.  In some cases, trades are ordered to be rolled back (reversed) by the exchange but this is usually reserved for institutional traders who make some sort of mistake.  This article below mentions day traders who bought at or near the bottom of the May 6, 2010 flash crash. This was before circuit breakers but I think it's a good story for someone looking to understand the finer workings of the electronic market.  http://www.marketwatch.com/story/book-takes-a-look-inside-professional-day-traders-1339513989350
A stop-loss order becomes a market order when a trade has occurred at or below the trigger price you set when creating the order. This means that you could possibly end up selling some or all of your position at a price lower than your trigger price. For relatively illiquid securities your order may be split into transactions with several buyers at different prices and you could see a significant drop in price between the first part of the order and the last few shares. To mitigate this, brokers also offer a stop-limit order, where you set not only a trigger price, but also a minimum price that you are will to accept for your shares. This reduces the risk of selling at rock bottom prices, especially if you are selling a very large position. However, in the case of a flash crash where other sellers are driving the price below your limit, that part of your order may never execute and you could end up being stuck with a whole lot of shares that are worth less than both your stop loss trigger and limit price. For securities that are liquid and not very volatile, either option is a pretty safe way to cut your losses. For securities that are illiquid and/or very volatile a stop-limit order will prevent you from cashing out at bottom dollar and giving away a bargain to lurkers hanging out at the bottom of the market, but you may end up stuck with shares you don't want for longer than originally planned. It's up to you to decide which kind of risk you prefer. why not just use a conditional order (http://www.investopedia.com/university/intro-to-order-types/conditional-orders.asp)? Like a one triggers one order? an order like this lets you place a buy order for the stock and if its executed another order is automatically placed. you could choose to let your second order be a stop order. so here's a company that offers stuff like this as an ex. (https://www.tradeking.com/education/tools/one-triggers-other-order) First you will need a plan stating three main points: You will have to decide what criteria you will use to answer these points. You might use Fundamental Analysis to find what to buy and Technical Analysis to decide when to buy and when to sell (your buy and sell triggers). Once you have a Trading Plan in place you would need to find a broker with conditional orders. You can place conditional buy orders to get in a trade (for example if the price moves above or below a target price). You can place conditional stop loss orders if your trade goes against you, and you can also place conditional profit target stops to automatically get out if rises to your desired profit target. You can place one, two or many conditional orders after hours which will enable you to trade without being in front of your screen all day long. Do not use a stop loss order as a long-term investor.  The arguments in favor of stop losses being presented by a few users here rely on a faulty premise, namely, that there is some kind of formula that will let you set your stop such that it won't trigger on day-to-day fluctuations but will trigger in time to protect you from a significant loss in a serious market downturn.  No such formula exists.  No matter where you set your stop, it is as likely to dump you from your investment just before it begins climbing again as it is to shield you from continued losses.  Each time that happens, you will have sold low and bought high, incurring trading fees into the bargain.  It is very unlikely that the losses you avoid in a bear market (remember, you still incur the loss up until your stop is hit; it's only the losses after that that you avoid) will make up the costs of false alarms. On top of that, once you have stopped out of your first investment choice, then what?  Will you reinvest in some other stock or fund?  If those investments didn't look good to you when you first set up your asset allocation, then why should they look any better now, just because your primary investment has dropped by some arbitrary[*] amount?  Will you park the money in cash while you wait for prices to bottom out?  The market bottom is only apparent in retrospect.  There is no formula for calling it in real time. Perhaps stop loss orders have their uses in active trading strategies, or maybe they're just chrome that trading platforms use to attract customers.  Either way, using them on long-term investments will just cost you money in the long run.  Forget the fancy order types, and manage your risk through your asset allocation.  The overwhelming likelihood is that you will get better performance, and you will spend less time worrying about your investments to boot. [*] Why are the stop levels recommended by the formulae invariably multiples of 5%?  Do the market gods have a thing for round numbers? "Yes, almost always. I trade some of the most illiquid single stock options, and I would be absolutely murdered if I didn't try to work orders between the bid/ask.  When I say illiquid, I mean almost non-existent: ~50 monthly contracts on ALL contracts for a given underlying.  Spreads of 30% or more. The only time you shouldn't try to work an order, in my opinion, is when you think you need to trade immediately (rare), if implied volatility (IV) has moved to such a degree that the market makers (MM) won't hit your order while they're offering fair IV (they'll sometimes come down to meet you at their ""real"" price to get the exchange's liquidity rebate), or if the bid/ask spread is a penny. For illiquid single stock options, you need to be extremely mindful of implied and statistical volatility.  You can't just try to always put your order in the middle.  The MMs will play with the middle to get you to buy at higher IVs and sell at lower. The only way you can hope that an order working below the bid / above the ask will get filled is if a big player overwhelms the MMs' (who are lined up on the bid and ask) current orders and hits yours with one large order.  I've never seen this happen. The only other way is like you said: if the market moves against you, the orders in front of yours disappear, and someone hits your order, but I think that defeats the intent of your question." I have done this, and the reason is to make sure that I don't run out of money in my account to place the order if there is an unexpected upswing in price. Suppose I have $1000 in my account and I want to buy 10 shares of ABCD that are currently at $99.  If the price doesn't change, then I am all set, but if the price goes up to $101 then I don't have sufficient funds to make the purchase. By placing a limit order at $100 I can ensure that I have enough money to place the order. In general, it is a rather unlikely scenario that it could happen, but placing the limit order is easy to do and it gives me peace of mind. I don't know what you mean about bypassing the queue. Futures exchanges are essentially auction houses facilitating a two-way auction. While they provide a venue for buyers and sellers to come together and transact (be that a physical venue such as a pit at the CME or an electronic network such as Globex), they don't actively seek out or find buyers and sellers to pair them together. The exchanges enable this process through an order book. As a futures trader you may submit one of two types of order to an exchange: Market Order - this is sent to the exchange and is filled immediately by being paired with a limit order.   Limit Order - this is placed on the books of the exchange at the price you specify. If other participants enter opposing market orders at this price, then their market order will be paired with your limit order. In your example, trader B wishes to close his long position. To do this he may enter a market sell order, which will immediately close his position at the lowest possible buy limit price, or he may enter a limit sell order, specifying the price at or above which he is willing to sell. In the case of the limit order, he will only sell and successfully close his position if his order becomes the lowest sell order on the book. All this may be a lot easier to understand by looking at a visual image of an order book such as the one given in the explanation that I have published here: Stop Orders for Futures Finally, not that as far as the exchange is concerned, there is no difference between an order to open and an order to close a position. They're all just 'buy' or 'sell' orders. Whether they cause you to reduce/exit a position or increase/establish a position is relative to the position you currently hold; if you're flat a buy order establishes a new position, if you're short it closes your position and leaves you flat. If you are the only one who puts in a large market buy order, then it would definitely push the price up. How much up would depend on how many would be willing to sell at what price point. It would also be possible that your trade will not get executed as there are no sellers. The same would be true if you put in a large sell order, with no buyers. The price would go down or trade not get executed as there aren't enough buyers.