High Level Alchemy (high alch, or alching) is a non-combat Magic spell used to convert items into coins at the best price a player could get from a specialty store. The amount of coins generated is normally 50% more than the general store or Low Alchemy gives.
High Level Alchemy is a popular way for mid and high level mages to train their magic level, by buying or making valuable items and nature runes, then casting the spell on the items. For example, a player might buy or make, yew longbows and cast High Level Alchemy on it. In this case, the player may consume approximately 290 coins per bow, or if they made them, make a profit. For Free Players, it is suggested that they High Alchemise Rune Square shields or adamant platebodies or Rune hatchet, as the total loss in coins for runes and Shield combined is only around 30 coins for the adamant platebody, and 75 coins for the Rune Square shields.
High Alchemy takes 5 game ticks, or about 3.0 seconds, to cast. It can be cast up to about 1185 times in one hour, although in practice many people only cast it 900-1100 times per hour. In the past, users had to time this spell casting so that they could maximize the number of alchs/hour (similarly to burying bones at double speed). However, JaGeX implemented a spell-cast queue for this spell, so players just have to cast it while the player is busy with it and the character will automatically queue one more cast.
In the past, Castle Wars was a favourite location for casting this spell, because the 20 minute time limit provided a reasonable amount of measurement. It was also enjoyable for the players to watch others fight. Due to player complaints about other players free-loading this way, this spell can no longer be cast inside a Castle Wars minigame. However, people still go along the walls of castle wars and cast alchemy spells, as you can still get somewhat of a show as you do it.
High (and Low) Alchemy can be cast on noted items, saving much withdrawal time from the bank, and simplifying the "click pattern". Also, it should be noted that High Alchemy only grants 65 experience, regardless of the rarity or expensiveness of the item you are alchemising. For example, high-alchemising a Bronze pickaxe grants no less experience than high-alchemising a Christmas cracker!
In practice, a player might prefer training High Level Alchemy by buying items from other players, which usually results in more coins being created than paid for the converted item. This is widely regarded as a major cause of inflation in the game.
Some items, such as rares, are worth far more on the street than in the store. As such, players are advised not to carry any rare items in their inventory, as accidental alchemising would cause a major loss for the player. An update on 9 November, 2009 stops this problem by warning the player if they try to cast high level alchemy on something worth 500k or more on the Grand Exchange.
Casting alchemy spells on stackable items will only alchemise one at a time; on RuneScape Classic, it alchemises the entire stack at once.
Note that it is generally not profitable to buy both the object and nature runes. The player should make either the nature runes (from pure essence) or the item in order to make a profit (making both will obviously give a larger profit). However, there are some instances in which about 40 coins can be made from this process.
On 19 May, 2009 Jagex updated low and high alchemy so that if a person clicks too early, the early click will not disrupt the alchemy process. 
One option to reduce the effort when using High Alchemy on hundreds or thousands of items in a session is to enable an accessibility feature of Microsoft Windows called MouseKeys. This allows the '5' key on the number pad to be used as a substitute for clicking the left mouse button. To enable the feature on Windows XP (also works on Vista and Windows 7) hold down the Left Shift, Left Alt and Numlock keys simultaneously to show the MouseKeys dialogue and then click setting-setting in lower part select when numlock is off then click the OK button. Press the Left Shift, Left Alt, and Numlock again and then click ok. The cursor can then be controlled by moving the mouse device as normal, or by holding down the numbers around '5' on the number pad, and clicking can be performed by pressing '5' on the number pad.
Mac OS X users can do the same thing by using Universal Access in System Preferences. In Mac OS X's Universal Access window, select the Mouse & Trackpad pane and then turn on Mouse Keys. With Mouse Keys turned on, you can navigate around using the number keys on the number pad and mouse-click with the 5 key. If using a laptop make sure you turn on the key pad by using the function key and hitting number lock (unless you have a number lock key which is on the 17" notepads), then you can use the '5' key which is usually the 'I' button or in that general area.
A similar feature can be turned on in Linux (Gnome or KDE) by pressing Left-Shift and Num Lock at the same time.
By positioning the items to be alchemised (in note form) at the left hand end of the fourth inventory row it is possible to repeatedly cast High Alchemy on the items by doing nothing more than pressing the '5' key at the correct intervals, with no mouse movement required.
All items in RuneScape have a fixed amount of coins that the High Level Alchemy spell yields (with the exception of quest items that cannot be alched). Some can turn a profit, but most will provide much less money than if they were sold to another player. There is a short, constantly updating, list of profitable high alchemy items here. See also a list of suggested items to cast High Level Alchemy on here.