Does a bid and ask price exist for indices like the S&P500?
The equation you show is correct, you've simply pointed out that you understand that you buy at the 'ask' price, and later sell at the 'bid.' There is no bid/ask on the S&P, as you can't trade it directly.  You have a few alternatives, however - you can trade SPY, the (most well known) S&P ETF whose price reflects 1/10 the value or VOO (Vanguard's offering) as well as others. Each of these ETFs gives you a bid/ask during market hours. They trade like a stock, have shares that are reasonably priced, and are optionable.  To trade the index itself, you need to trade the futures. S&P 500 Futures and Options is the CME Group's brief info guide on standard and mini contracts.  Welcome to SE. You can trade an index by using a Contract For Difference, or CFD. Various brokers offer this method and the spreads are quite low. They tend to widen outside of market hours, and not all brokers offer the same spreads. I would look for a broker that offers the lowest spread on the index you are interested in. You should also do your due diligence and check they are regulated by the relevant authority  pertaining to their territory, eg FSA for uk Bid and ask prices of stocks change not just daily, but continuously.  They are, as the names suggest, what price people are asking for to be willing to sell their stock, and how much people are bidding to be willing to buy it at that moment. Your equation is accurate in theory, but doesn't actually apply.  The bid and ask prices are indicators of the value of the stock, but the only think you care about as a trader are what you actually pay and sell it for.  So regardless of the bid/ask the equation is: Since you cannot buy an index directly (index, like indicator) it doesn't make sense to discuss how much people are bidding or asking for it.  Like JoeTaxpayer said, you can buy (and therefore bid/ask) for ETFs and funds that attempt to track the value of the S&P 500.
The best ask is the lowest ask, and the best bid is the highest bid.  If the ask was lower than the bid then they crossed, and that would be a crossed market and quickly resolved. So the bid will almost always be cheaper than the ask. A heuristic is that a bid is the revenue of the stock at any given time while the ask is the cost, so the market will only ever offer a profit to itself not to the liquidity seeker. If examining the book vertically, all orders are usually sorted descending. Since the best ask is the lowest ask, it is on the bottom of the asks, and vice versa for the best bid. The best bid & best ask will be those closest since that's the narrowest spread and price-time priority will promise that a bid that crosses the asks will hit the lowest ask, the best possible price for the bidder and vice versa for an ask that crosses the best bid. "- In a quote driven market, must every investor trade with a market        maker? In other words, two parties that are both not market makers        cannot trade between themselves directly? In a way yes, all trades go through a market maker but those trades can be orders put in place by a ""person"" IE: you, or me. - Does a quote driven market only display the ""best"" bid and ask        prices proposed by the market makers? In other words, only the        highest bid price among all the market makers is displayed, and        other lower bid prices by other market makers are not? Similarly,        only the lowest ask price over all market makers is displayed, and        other higher ask prices by other market makers are not? No, you can see other lower bid and higher ask prices. - In a order-driven market, is it meaningful to talk about ""the current stock price"", which is the price of last transaction? Well that's kind of an opinion. Information is information so it won't be bad to know it. Personally I would say the bid and ask price is more important. However in the real world these prices are changing constantly and quickly so realistically it is easier to keep track of the quote price and most likely the bid/ask spread is small and the quote will fall in between. The less liquid a security is the more important the bid/ask is. -- This goes for all market types. - For a specific asset, will there be several transactions happened at    the same time but with different prices? Today with electronic markets, trades can happen so quickly it's difficult to say. In the US stock market trades happen one at a time but there is no set time limit between each trade. So within 1 second you can have a trade be $50 or $50.04. However it will only go to $50.04 when the lower ask prices have been exhausted.   - Does an order driven market have market makers?  By definition, no. - What are some examples of quote driven and order driven financial markets, in which investors are commonly trading stocks and derivatives, especially in U.S.? Quote driven market: Bond market, Forex. Order driven market: NYSE comes from an order driven market but now would be better classified as a ""hybird market"" Conclusion: If you are asking in order to better understand today's stock markets then these old definitions of Quote market or Order market may not work. The big markets in the real world are neither. (IE: Nasdaq, NYSE...) The NYSE and Nasdaq are better classified as a ""hybird market"" as they use more then a single tactic from both market types to insure market liquidity, and transparency. Markets these days are strongly electronic, fast, and fairly liquid in most cases. Here are some resources to better understand these markets: An Introduction To Securities Markets The NYSE And Nasdaq: How They Work Understanding Order Execution" If you look at a trade grid you can see how this happens. If there are enough bids to cover all shares currently on the sell side at a certain price, those shares will be bought and increased price quotes will be shown for the bids and ask. If there are enough bids to cover this price, those will get bought and higher prices will be shown and this process will repeat until the sell side has more power than the buy side. It seems like this process is going on all day long with momentum either on the upside or downside. But I think that much of this bidding and selling is automatic and is being done by large trading firms and high tech computers. I also feel that many of these bids and asks are already programmed to appear once there is a price change. So once one price gets bought, computers will put in higher bids to take over asks. It's like a virtual war between trading firms and their computers. When more money is on the buy side the stock will go up, and vice versa. I sort of feel like this high-frequency trading is detrimental to the markets and doesn't really give everyone a fair shot. Retail investors do not have the resources and knowledge in order to do this sort of high frequency trading. It also seems to go against certain free market principles in my opinion. A market downturn is a great opportunity to pick up valuable stocks at low prices but do note that this is quite a risky proposition and requires abundant research. But again, as is mentioned in previous answers, the start of the downturn is evident only in hindsight, and it is always advisable to have a balanced/diversified portfolio, risking only what you can afford to lose. In any case, indices like Dow Jones, NASDAQ, S&P500 are calculated on the basis of only a few companies that broadly represent their individual sectors. If the share is listed on a stock exchange that creates liquidity and orderly sales with specialist market makers, such as the NYSE, there will always be a counterparty to trade with, though they will let the price rise or fall to meet other open interest.  On other exchanges, or in closely held or private equity scenarios, this is not necessarily the case (NASDAQ has market maker firms that maintain the bid-ask spread and can do the same thing with their own inventory as the specialists, but are not required to by the brokerage rules as the NYSE brokers are).  The NYSE has listing requirements of at least 1.1 million shares, so there will not be a case with only 100 shares on this exchange. "Market makers (shortened MM) in an exchange are generally required to list both a bid and ask price to allow both buyers and sellers to trade and keep the market moving.  However, a more general idea of a MM may includes companies off an exchange (say large banks acting as broker/dealers in an over-the-counter market) are not required to give a simultaneous bid/ask, but often will on request.  So, it might depend on where you are getting this data but likely the bid/ask was quoted simultaneously. An exchange, like the NASDAQ for instance, may have multiple MMs for a given market.  The ""market"" spread will be from the highest bid to the lowest ask over all the MMs.  The highest bid and lowest ask may come from different MMs and any particular MM often will have a larger spread. The size of the spread gives a rough idea of how much a MM is trying to make off of a ""round trip"" trade (buying than immediately selling to someone else or selling than immediately buying from someone else).  Of course, immediate round-trip trades are not always possible and there are many other complications.  However, half the spread is a rough indicator of how much they hope to make off of a single trade." "The S&P 500 index is maintained by S&P Dow Jones Indices, a division of McGraw Hill Financial. Changes to the index are made periodically, as needed.  For Facebook, you'll find it mentioned in this December 11, 2013 press release (PDF).  Quote: New York, NY, December 11 , 2013 – S&P Dow Jones Indices will make the following changes to the S&P 100, S&P 500, MidCap 400   and S&P SmallCap 600 indices after the close of trading on Friday,   December 20: You can find out more about the S&P 500 index eligibility criteria from the S&P U.S. Indices methodology document (PDF).  See pages 5 and 6: Market Capitalization - [...] Liquidity - [...] Domicile - [...] Public Float - [...] Sector Classification - [...] Financial Viability - Usually measured as four consecutive quarters of positive as   reported earnings. [...] Treatment of IPOs - Initial public offerings should be seasoned for 6 to 12 months   before being considered for addition to an index. Eligible Securities - [...]   [...]   Changes to the U.S. indices other than the TMIX are made as needed,   with no annual or  semi-annual reconstitution. [...] LabCorp may have a smaller market cap than Facebook, but Facebook didn't meet all of the eligibility criteria – for instance, see the above note about ""Treatment of IPOs"" – until recently. Note also that ""Initial public offerings should be seasoned for 6 to 12 months"" implies somebody at S&P makes a decision as to the exact when. As such, I would say, no, there is no ""simple rule or formula"", just the methodology above as applied by the decision-makers at S&P." In terms of pricing the asset, this functions in exactly the same way as a regular sell, so bids will have to be hit to fill the trade. When shorting an equity, currency is not borrowed; the equity is, so the value of per share liability is equal to it's last traded price or the ask if the equity is illiquid. Thus when opening a short position, the asks offer nothing to the process except competition for your order getting filled. Part of managing the trade is the interest rate risk.  If the asks are as illiquid as detailed in the question, it may be difficult even to locate the shares for borrowing. As a general rule, only illiquid  equities or those in free fall may be temporarily unable for shorting. Interactive Brokers posts their securities financing availabilities and could be used as a proxy guide for your broker.