(Reuters) - Since the start of this year, oil prices have risen by around 40 percent to an all-time high of more than $139 a barrel.
Over the same period the fundamentals of supply and demand have shifted far less dramatically, prompting comment the market has been pushed higher by a wave of speculation.
It is very hard to prove to what extent the price is driven by speculators, as opposed to those with exposure to physical oil, because there is a lack of clear data on who is buying oil and other commodities.
The most prominent of the available data is from the Commodity Futures Trading Commission (CFTC), which distinguishes between commercial players (including oil, mining companies and other producers of raw materials) and non-commercial players, often considered synonymous with speculators.
These statistics do not tell the full story because the categories are blurred. For instance, banks that hedge exposure to the oil price because they loan money to oil companies, also have involvement in commodity indexes.
The indexes are used by long-term investors, such as pension funds and insurers, to gain access to the commodities complex.
Another major flaw in the CFTC data is that it only covers U.S. regulated trade.
British regulators do not release figures on market participants.
The following is a list of the players on the commodities markets apart from the resource companies.
Hedge funds invest using aggressive strategies, including selling short. They stand in contrast to the more closely-regulated U.S. mutual funds, which also invest in assets on behalf of institutions or individuals, but by using more conservative strategies.
Global macro hedge funds take directional bets in stock, bond, currency and commodity markets using economic trends.
Global macro hedge funds probably have the widest of all hedge fund strategies. Their managers can take positions in any market or instrument anywhere in the world.
George Soros and Paul Tudor Jones are among the more famous macro hedge fund managers.
Those wary of the high-risk strategies of hedge funds could choose to use a fund of funds, which spreads the risk by investing in a range of hedge funds, including those specialized in commodities.
Pensions and insurers are a relatively recent arrival on the commodity markets.
Traditionally cautious players who preferred to gain commodity exposure by buying into the equities of raw material companies, they have become a significant presence on commodity futures markets during the current sustained rally begun around 2002.
They have begun to experiment with active and passive positions, but traditionally have taken long-only positions in commodity markets.
A high-net worth individual is generally defined as an individual with investable assets, excluding property, of more than $1 million.
Institutions and high-net worth individuals have formed the bulk of commodity investment, although analysts say interest from smaller retail investors has increased.
Commodity ISAs or Exchange-Traded Funds particularly cater for this market.
An Individual Savings Account (ISA) is a British financial product designed for investment and savings with a favorable tax status.
Reporting by Barbara Lewis and Santosh Menon