NEW YORK ( TheStreet)-- Shadow banks -- entities that provide credit as banks do but aren't subject to the same types of regulations -- were arguably at the center of the 2008 financial crisis. However, while rules for banks have been tightened since 2008, most experts agree that little if anything has been done to regulate shadow banks more closely. "Nothing has changed in terms of the regulatory structure of shadow banking. It's certainly the case that top regulators and analysts are much more aware now of problems that could go wrong and we don't have all of these very shaky asset backed securities floating around that would lead to the kind of meltdown that we saw a few years ago," says Yale University professor Andrew Metrick. However, he adds, "the main pillars of the money market mutual fund system, securitization and repo are not regulated any differently than they were in 2008," While Metrick and his Yale colleague Gary Gorton focus on those three main pillars, other experts have argued for a broader definition. An essay by Boston College finance professor Ed Kane, essentially defines shadow banks as entities that have persuaded their customers that they will be bailed out if necessary by a government--even though they are not formally protected by government guarantees. New York University professor Viral Acharya, meanwhile, argues that governments themselves are shadow banks. "By allowing excessive competition, providing downside guarantees and encouraging risky lending for populist schemes, governments can create periods of intense economic activity fueled by credit booms. This way, governments effectively operate as "shadow banks" in the financial sector. Such a government role appears to have been at the center of recent boom and bust cycles and continues to present a threat to financial stability," Acharya wrote in a paper published on the Federal Reserve's website. Shadow banks, in other words, aren't always easy to define and they often refer to markets rather than specific entities or institutions. What follows is a combination of both shadow bank markets, as well as institutions that--while not themselves shadow banks--have substantial exposure to the shadow banking system.