Why do stock or commodity prices sometimes rise suddenly just before market close?
This is often the case where traders are closing out short positions they don't want to hold overnight, for a variety of reasons that matter to them.  Most frequently, this is from day traders or high-frequency traders settling their accounts before the markets close.
"Some technical indicators (e.g. Williams %R) indicate whether the market is overbought or oversold.   ... Every time a stock or commodity is bought, it is also sold. And vice versa. So how can anything ever be over-bought or over-sold? But I'm sure I'm missing something. What is it? You're thinking of this as a normal purchase, but that's not really how equity markets operate. First, just because there are shares of stock purchased, it doesn't mean that there was real investor buyer and seller demand for that instrument (at that point in time).  Markets have dedicated middlemen called Market Makers, who are responsible to make sure that there is always someone to buy or sell; this ensures that all instruments have sufficient liquidity.  Market Makers may decide to lower their bid on a stock based on a high number of sellers, or raise their ask for a high number of buyers. During an investor rush to buy or sell an instrument (perhaps in response to a news release), it's possible for Market Makers to accumulate a large number of shares, without end-investors being involved on both sides of the transaction.  This is one example of how instruments can be over-bought or over-sold. Since Williams %R creates over-bought and over-sold signals based on historical averages of open / close prices, perhaps it's better to think of these terms as ""over-valued"" and ""under-valued"".  Of course, there could be good reason for instruments to open or close outside their expected ranges, so Williams %R is just a tool to give you clues... not a real evaluation of the instrument's true value." Like others have already said, it may cause an immediate dip due to a large and sudden move in shares for that particular stock. However, if there is nothing else affecting the company's financials and investors perceive no other risks, it will probably bounce back a bit, but not back to the full value before the shares were issued. Why? Whenever a company issues more stock, the new shares dilute the value of the current shares outstanding, simply because there are now more shares of that stock trading on the market; the Earnings Per Share (EPS) Ratio will drop since the same profit and company value has to be spread across more shares. Example:  If a company is valued at $100 dollars and they have 25 shares outstanding, then the EPS ratio equates to $4 per share (100/25 = 4).  If the company then issues more shares (stock to employees who sell or keep them), let's say 25 more shares, then shares outstanding increase to 50, but the company's value still remains at $100 dollars. EPS now equates to $2 per share (100/50 = 2). Now, sometimes when shareholders (especially employees...and especially employees who just received them) suddenly all sell their shares, this causes a micro-panic in the market because investors believe the employees know something bad about the company that they don't. Other common shareholders then want to dump their holdings for fear of impending collapse in the company. This could cause the share price to dip a bit below the new diluted value, but again if no real, immediate risks exist, the price should go back up to the new, diluted value. Example 2: If EPS was at $4 before issuing more stock, and then dropped to $2 after issuing new stock, the micro-panic may cause the EPS to drop below $2 and then soon rebound back to $2 or more when investors realize no actual risk exists. After the dilution phase plays out, the EPS could actually even go above the pre-issuing value of $4 because investors may believe that since more stock was issued due to good profits, more profits may ensue. Hope that helps! "Supply and Demand, pure and simple! There are two basic forms of this - a change in the quantity demanded/supplied at any given price, and a true change in the amount of demand/supply itself. Please note that this can be distinct from the underlying change in the value of the company and/or its expected future cash flows, which are a function of both financial performance and future expectations. If more people want the stock that are willing to sell it at a given price at a given point in time, sellers will begin to offer the stocks at higher prices until the market is no longer willing to bear the new price, and vice versa. This will reduce the quantity of stocks demanded by buyers until the quantity demanded and the quantity supplied once again reach an equilibrium, at which point a transaction occurs. Because people are motivated to buy and sell for different reasons at different times, and because people have different opinions on a constant flow of new information, prices change frequently. This is one of the reasons why executives of a recent IPO don't typically sell all of their stock at once. In addition to legal restrictions and the message this would send to the market, if they flooded the market with additional quantities of stock supplied, all else being equal, since there is no corresponding increase in the quantity demanded, the price would drop significantly. Sometimes, the demand itself for a company's stock shifts. Unlike a simple change in price driven by quantity supplied versus quantity demanded, this is a more fundamental shift. For example, let's suppose that the current demand for rare earth metals is driven by their commercial applications in consumer electronics. Now if new devices are developed that no longer require these metals, the demand for them will fall, regardless of the actions of individual buyers and sellers in the market. Another example is when the ""rules of the game"" for an industry change dramatically. Markets are behavioral. In this sense prices are most directly driven by human behavior, which hopefully is based on well-informed opinions and facts. This is why sometimes the price keeps going up when financial performance decreases, and why sometimes it does not rise even while performance is improving. This is also why some companies' stock continues to rise even when they lose huge sums of money year after year. The key to understanding these scenarios is the opinions and expectations that buyers and sellers have of that information, which is expressed in their market behavior." UNG    United States Natural Gas Fund  Natural Gas USO    United States Oil Fund  West Texas Intermediate Crude Oil UGA    United States Gasoline Fund     Gasoline DBO    PowerShares DB Oil Fund     West Texas Intermediate Crude Oil UHN    United States Heating Oil Fund  Heating Oil I believe these are as close as you'd get. I'd avoid the double return flavors as they do not track well at all.  Update - I understand James' issue. An unmanaged single commodity ETF (for which it's impractical to take delivery and store) is always going to lag the spot price rise over time.   And therefore, the claims of the ETF issuer aside, these products will almost certain fail over time. As shown above, When my underlying asset rises 50%, and I see 24% return, I'm not happy. Gold doesn't have this effect as the ETF GLD just buys gold, you can't really do that with oil. This would be a nice Raspberry Pi project for Mathematica, which comes bundled free on the Raspbian OS. You can program it up and leave it running.  It's not expensive and doesn't use much power. A program to monitor stock prices or volume could be written as simply as :-  This checks the volume of trades of Oct 2014 US crude oil futures every 30 seconds and sends an email if the volume jumps by more than 100. The financial data in this example is curated from Yahoo.  If specific data is not available or not updated frequently enough, if you can find an alternative online data source it's usually possible read the data in.  For example, this is apparently real-time data :- {Crude Oil, 92.79, -0.67, -0.71%} After leaving the above program running while writing this the volume of trades has risen like so :-  Edit I just set this running on a Raspberry Pi.  I had to use gmail for the email setup as described in this post: Configuring Mathematica to send email from a notebook.  Anyway, it's working.  Hope I don't get inundated with emails. ;-) "There are many reasons. Here are just some possibilities: The stock has a lot of negative sentiment and puts are being ""bid up"". The stock fell at the close and the options reflect that. The puts closed on the offer and the calls closed on the bid. The traders with big positions marked the puts up and the calls down because they are long puts and short calls. There isn't enough volume in the puts or calls to make any determination - what you are seeing is part of the randomness of a moment in time." "This could be another reason.  ""Companies buy their own stock in the market place to reduce the number of shares outstanding, and thus boosts the earnings per share. It also boosts the stock price, which benefits management that has stock options. "" Taken from this article. http://www.forbes.com/sites/investor/2014/01/06/the-most-reliable-indicator-of-an-approaching-market-top/ and this article ""Why are stock rising?"" may help as well.  http://www.forbes.com/sites/investor/2013/12/23/why-are-stocks-rising/" Here are some plausible reasons why markets might continue to close: