Why is the bid-ask spread considered a cost?
Your assets are marked to market. If you buy at X, and the market is bidding at 99.9% * X then you've already lost 0.1%. This is a market value oriented way of looking at costs. You could always value your assets with mark to model, and maybe you do, but no one else will.  Just because you think the stock is worth 2*X doesn't mean the rest of the world agrees, evidenced by the bid.  You surely won't get any margin loans based upon mark to model.  Your bankers won't be convinced of the valuation of your assets based upon mark to model. By strictly a market value oriented way of valuing assets, there is a bid/ask cost. more clarification Relative to littleadv, this is actually a good exposition between the differences between cash and accrual accounting. littleadv is focusing completely on the cash cost of the asset at the time of transaction and saying that there is no bid/ask cost.  Through the lens of cash accounting, that is 100% correct. However, if one uses accrual accounting marking assets to market (as we all do with marketable assets like stocks, bonds, options, etc), there may be a bid/ask cost.  At the time of transaction, the bids used to trade (one's own) are exhausted.  According to exchange rules that are now practically uniform: the highest bid is given priority, and if two bids are bidding the exact same highest price then the oldest bid is given priority; therefore the oldest highest bid has been exhausted and removed at trade. At the time of transaction, the value of the asset cannot be one's own bid but the highest oldest bid leftover.  If that highest oldest bid is lower than the price paid (even with liquid stocks this is usually the case) then one has accrued a bid/ask cost. "This is a misconception. One of the explanations is that if you buy at the ask price and want   to sell it right away, you can only sell at the bid price. This is incorrect. There are no two separate bid and ask prices. The price you buy (your ""bid"") is the same price someone else sells (their ""sell""). The same goes when you sell - the price you sell at is the price someone else buys. There's no spread with stocks. Emphasized it on purpose, because many people (especially those who gamble on stock exchange without knowing what they're doing) don't understand how the stock market works. On the stock exchange, the transaction price is the match between the bid price and the ask price. Thus, on any given transaction, bid always equals ask. There's no spread. There is spread with commodities (if you buy it directly, especially), contracts, mutual funds and other kinds of brokered transactions that go through a third party. The difference (spread) is that third party's fee for assuming part of the risk in the transaction, and is indeed added to your cost (indirectly, in the way you described). These transactions don't go directly between a seller and a buyer. For example, there's no buyer when you redeem some of your mutual fund - the fund pays you money. So the fund assumes certain risk, which is why there's a spread in the prices to invest and to redeem. Similarly with commodities: when you buy a gold bar - you buy it from a dealer, who needs to keep a stock. Thus, the dealer will not buy from you at the same price: there's a premium on sale and a discount on buy, which is a spread, to compensate the dealer for the risk of keeping a stock." "As an aside, on most securities with a spread of the minimum tick, there would be no bid ask spread if so-called ""locked markets"", where the price of the best bid on one exchange is equal to the price of the best ask on another, were permitted. It is currently forbidden for a security to have posted orders having the same price for both bid and ask even though they're on different exchanges. Option spreads would narrow as well as a result."
"ETFs purchases are subject to a bid/ask spread, which is the difference between the highest available purchase offer (""bid"") and the lowest available sell offer (""ask""). You can read more about this concept here. This cost doesn't exist for mutual funds, which are priced once  per day, and buyers and sellers all use the same price for transactions that day. ETFs allow you to trade any time that the market is open. If you're investing for the long term (which means you're not trying to time your buy/sell orders to a particular time of day), and the pricing is otherwise equal between the ETF and the mutual fund (which they are in the case of Vanguard's ETFs and Admiral Shares mutual funds), I would go with the mutual fund because it eliminates any cost associated with bid/ask spread." "Both prices are quotes on a single share of stock. The bid price is what buyers are willing to pay for it. The ask price is what sellers are willing to take for it. If you are selling a stock, you are going to get the bid price, if you are buying a stock you are going to get the ask price. The difference (or ""spread"") goes to the broker/specialist that handles the transaction." How would this trade behave IRL? I don't know how the simulation handles limit orders and bid/ask spreads to know it's feasible, but buying at 4.04 when the current ask is 8.00 seems unlikely.  That would mean that all other sell orders between 8.00 and 4.04 were fulfilled, which means that there were very few sellers or that sell pressure spiked, both of which seem unlikely. In reality, it seems more likely that your order would have sat there until the ask dropped to $4.04 (if it ever did), and then you'd have to wait until the bid rose to $7.89 in order to sell them at that price. However, that kind of swing in option prices in not unrealistic.  Options near at-the-money tend to move in price at about 50% of the change in the underlying, so if amazon suddenly dropped by $5, the option price could drop by $2.60 (from 6.66 to $4.04), and then rise back to $7.89 if the price rose $8 (which would be 1% swing and not unheard of intra-day). But it sounds like you got very lucky (or the simulation doesn't handle option trading realistically) - I've traded options in the past and have had some breaks similar to yours.  I've also had bad breaks where I lost my entire investment (the options expire out-of-the money). So it should be a very limited part of your portfolio, and probably only used for risk management (e.g. buying put options to lock in some gains but keeping some upside potential). "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 believe that the Wikipedia article is attempting to draw a difference between dollar cost averaging as a result of investing when money becomes available (i.e. 401k contributions from your paycheck) and spreading out a lump sum investment. For the former you want to continuously invest as the money becomes available following your predetermined plan for allocation. For the later it may be reasonable to consider the market as you decide how and the timing for investing. "Both prices are quotes on a single share of stock. The bid price is what buyers are willing to pay for it. The ask price is what sellers are willing to take for it. If you are selling a stock, you are going to get the bid price, if you are buying a stock you are going to get the ask price. The difference (or ""spread"") goes to the broker/specialist that handles the transaction." The price of a share has two components: Bid: The highest price that someone who wants to buy shares is willing to pay for them. Ask: The lowest price that someone who has a share is willing to sell it for. The ask is always higher than the bid, since if they were equal the buyer and seller would have a deal, make a transaction, and that repeats until they are not equal. For stock with high volume, there is usually a very small difference between the bid and ask, but a stock with lower volume could have a major difference. When you say that the share price is $100, that could mean different things. You could be talking about the price that the shares sold for in the most recent transaction (and that might not even be between the current bid and ask), or you could be talking about any of the bid, the ask, or some value in between them. If you have shares that you are interested in selling, then the bid is what you could immediately sell a share for. If you sell a share for $100, that means someone was willing to pay you $100 for it. If after buying it, they still want to buy more for $100 each, or someone else does, then the bid is still $100, and you haven't changed the price. If no one else is willing to pay more than $90 for a share, then the price would drop to $90 next time a transaction takes place and thats what you would be able to immediately sell the next share for. One reason this happens is due to dividends. If the dividend amount is greater than the time value left on a call, it can make sense to exercise early to collect the dividend.  Deep in the money puts also may get exercised early. There's usually little premium on a deep in the money put and the spread on the bid-ask might erase what little premium there is. If you have stock worth $5,000 but own puts on them that will give you $50,000 upon exercise (and no spread to worry about), the interest you can gain on the $50k might be more than the little to no time value left on the position... even at several weeks to expiration.