Acetone peroxide
Acetone peroxide (triacetone triperoxide, peroxyacetone, TATP, TCAP) is an organic peroxide and a primary high explosive. It takes the form of a white crystalline powder with a distinctive acrid smell.
It is highly susceptible to heat, friction, and shock. For its instability, it has been called the "Mother of Satan".  It has perhaps sprung into notoriety due to its alleged use in the July 2005 London bombings and has also been reported as the explosive favored by suspects arrested on August 10, 2006 who allegedly intended to destroy aeroplanes flying from the United Kingdom to the United States.
Acetone peroxide was discovered in 1895 by Richard Wolffenstein. He was the first chemist who used inorganic acids as a catalyst. He was also the first researcher who received a patent for using the peroxide as an explosive compound. In 1900 Bayer and Villiger described in some articles in the same journal the first synthesis of the dimer and used acids for the synthesis of both peroxides too. Information about it including the relative proportions of monomer, dimer, and trimer is also available an article of Milas and Golubovic. Other sources include crystal structure and 3d analysis in "The Chemistry of Peroxides" edited by Saul Patai (pp. 3967), as well as the "Textbook of Practical Organic Chemistry" by Vogel.

# Chemistry
Also known as "peroxyacetone", acetone peroxide most commonly refers to the cyclic trimer TCAP (tri-cyclic acetone peroxide, or tri-cyclo), also called triacetone triperoxide (TATP), obtained by a reaction between hydrogen peroxide and acetone in an acid-catalyzed nucleophilic addition. The cyclic dimer (C6H12O4) and open monomer and dimer are also formed, but under proper conditions the cyclic trimer is the primary product. A tetrameric form was also described. In mildly acidic or neutral conditions, the reaction is much slower and produces more monomeric organic peroxide than the reaction with a strong acid catalyst. Due to significant strain of the chemical bonds in the dimer and especially the monomer, they are even more unstable than the trimer.
At room temperature, the trimeric form slowly sublimes, reforming as larger crystals of the same peroxide.
Acetone peroxide is notable as a high explosive not containing nitrogen. This is one reason why it has become popular with terrorists, as it can pass through scanners designed to detect nitrogenous explosives.
TCAP generally burns when ignited, unconfined, in quantities less than about 2 grams. More than 2 grams will usually detonate when ignited; smaller quantities might detonate when even slightly confined. Completely dry TCAP is much more prone to detonation than the fresh product still wetted with water or acetone. The oxidation that occurs when burning is:
Theoretical examination of the explosive decomposition of TCAP, in contrast, predicts in "formation of acetone and ozone as the main decomposition products and not the intuitively expected oxidation products."  But even in 1943 German researcher(s) described in the case of detonation of the trimer the formation of formaldehyde which is clearly a result of a fragmentation of primary formed oxyradicals. This result is in good agreement with the results of 60 years of the study of controlled decompositions in various organic peroxides. It is the rapid creation of gas from a solid that creates the explosion. Very little heat is created by the explosive decomposition of TCAP. Recent research describes TCAP decomposition as an entropic explosion.
The extreme shock, heat, and friction sensitivity are due to the instability of the molecule. Big crystals, found in older mixtures, are more dangerous, as they are easier to shatter  and initiate  than small ones.
Due to the cost and ease with which the precursors can be obtained, acetone peroxide is commonly manufactured by those without the resources needed to manufacture or buy more sophisticated explosives. When the reaction is carried out without proper equipment the risk of an accident is significant.
There is a common myth that the only "safe" acetone peroxide is the trimer, made at low temperatures:
"The mixture must be kept below 10 degrees Celsius. If the crystals form at this temperature, it forms the isomer called tricycloacetone peroxide, which is relatively stable and safe to handle. If the crystals form above this temperature, the dimerric form, called dicycloacetone peroxide. This isomer is much more unstable, and could go off at the touch, making it not safe enough to be considered a practical explosive. As long as the temperature is kept below 10 degrees Celsius, then there is little to worry about."
In reality, the acid-catalyzed peroxidation of acetone always produces a mixture of dimeric and trimeric forms.
The trimer is the more stable form, but not much more so than the dimer. All forms of acetone peroxide are very sensitive to initiation. Organic peroxides are sensitive, dangerous explosives, due their sensitivity they are rarely used by well funded militaries. Even for those who synthesize explosives as a hobby there are far safer explosives with syntheses nearly as simple as that of acetone peroxide.
Tetrameric acetone peroxide is more chemically stable (heating to 120C for 4 hours), but despite this it's still a very dangerous primary explosive. It can be prepared using Tin(IV) chloride (without acid present) as a catalyst with up to 40% yield if a particular radical inhibitor is added.

# Industrial occurrence
Acetone peroxides are common and unwanted by-products of oxidation reactions, eg. those used in phenol syntheses. Due to their explosivity, they are hazardous. Numerous methods are used to reduce their production - shifting the pH to more alkaline, adjusting the reaction temperature, or adding a soluble copper(II) compound.
Acetone peroxide and benzoyl peroxide are used as flour bleaching agents to bleach and "mature" flour.
Ketone peroxides, including acetone peroxide, methyl ethyl ketone peroxide, and benzoyl peroxide, find applications as initiators for polymerization reactions of eg. silicone or polyester resins, often encountered when making fiberglass. For these uses, the peroxides are typically in the form of a dilute solution in an organic solvent, however, even commercial products with higher concentrations of organic peroxides can form crystals around the lid when older, making the can shock-sensitive. Methyl ethyl ketone is more common for this purpose, however, as it is stable in storage.

# Accidental byproduct
Acetone peroxide can also occur accidentally, when suitable chemicals are mixed together. For example, when methyl ethyl ketone peroxide is mixed with acetone when making fiberglass, and left to stand for some time, or when a mixture of peroxide and hydrochloric acid from printed circuit board etching (the FeCl3 method is less smelly, more accurate, but slower) is mixed with waste acetone from cleaning the finished board and allowed to stand. While amounts obtained this way are typically much smaller than from intentional production, they are also less pure and prepared without cooling, and hence very unstable.
It is also a hazardous by-product of isosafrole oxidation in acetone, a step in the synthesis of MDMA.

# Clandestine usage
TATP has been identified in explosive devices in a number of cases involving terrorists. Richard Reid, who attempted to down  American Airlines Flight 63 with a bomb concealed in his shoe, employed a device containing plastic explosive with a TATP trigger. It is also believed that acetone peroxide was used as the explosive in the 7 July 2005 London bombings. On September 5, 2006, homemade TATP was found during the arrest of seven suspected terrorists in Vollsmose, a neighborhood in the Danish city Odense, as well as on September 4, 2007, during the arrest of eight suspected Al-Queda collaborators in Copenhagen, Denmark. In addition, the participants in the 2006 transatlantic aircraft plot may have planned to use TATP as the liquid bombs, mixed in aeroplane lavatories, that would destroy U.S. airliners flying from London to the United States. Nevertheless, it has been pointed out that it's highly questionable whether such a plot could have been executed, due to the supplies needed, the smell mixing would create, and the time it would take to prepare without drawing suspicion from passengers and the flight crew.