The U.S. Securities and Exchange Commission issued a concept paper today that initiates a broad review of equity market structure issues.

The commission voted unanimously to issue a concept release seeking public comment on a variety of market structure issues, such as high frequency trading, co-locating trading terminals, and dark pools. The SEC says that the purpose of the review is to ensure that the current market structure serves the interests of long-term investors, including both retail and institutional investors.

“At the commission, we must continually assess how changes in the market are affecting investors,” said SEC chairman Mary Schapiro. “We must try to understand how these changes may impact the markets in the future, so we can steer clear of any unnecessary risks to investors.”

The commission will assess how all types of individual investors and all sizes of institutional investors are faring in the current market framework. It is also assessing whether the current market structure promotes capital formation in companies with varying levels of market capitalization.

The ongoing review of market structure issues has already led to several rulemaking proposals in recent months, including: a proposed ban on flash orders, a proposal that would strengthen transparency requirements for non-public trading interest, and today the commission proposed for public comment a new market structure initiative to strengthen the risk management controls of broker-dealers that provide market access.

The commission says it intends to use the comments it receives on the concept release to help determine whether additional regulatory measures are needed to improve the current equity market structure.