As federal regulators and Congress zero in on Apple, Google, Facebook, and Amazon, they’re about to encounter one of the most difficult rulemaking challenges in US history. The tech giants don’t fit neatly into the existing model for antitrust action since many of their services are available for free, making any consumer harm they may or may not have done difficult to grasp and quantify. And perhaps more vexingly, they are constantly shifting shape, adding new business lines with regularity to keep pace with a fast-changing technology industry. In Washington, it’s going to be hard to figure out where to even begin.

“One of the things we’ve seen in the past with regulation is by the time the courts catch up to regulating the industry often new industries emerge,” Rep. Ro Khanna, who represents a large slice of Silicon Valley, told BuzzFeed News. “Of course, we need regulation, but it has to be thoughtful regulation and well-crafted regulation.”

Consider the moving target the Federal Trade Commission will encounter when it examines Amazon. The FTC won’t just be dealing with a retailer, but an outsourced logistics provider, a grocer, a cloud services clearinghouse, a hardware manufacturer, and a voice search company. And as the FTC works to get its head around these business lines, Amazon will inevitably expand into more — the company is rumored to be debuting a home robot this year.

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Amazon is ferociously aggressive in many of its business lines, yet it faces fierce competition in nearly all of them. There’s Amazon.com vs. Walmart, Whole Foods vs. the broader grocery industry, AWS vs. Microsoft Azure, Amazon Echo vs. Google Home.

With such a diverse set of businesses, Amazon will make it hard for regulators to reign in the “bigness” many are hoping it will tackle. Amazon and its fellow tech giants are nothing like the Bell Telephone Company or Standard Oil, which grew dominant by finding a core advantage and defending it at all costs. They have instead built their empires through continual reinvention, and they are far more nimble than their corporate predecessors. Regulators will therefore have to comb through each business line, consider the market dynamics in each, and toe the line between policing anti-competitive behavior and picking winners and losers.

“I think there is a lot of appetite for letting the market sort lots of things out,” Robert Seamans, an associate professor at NYU’s Stern School of Business who spent a year as a senior economist at the White House Council of Economic Advisers, told BuzzFeed News. “We don’t want to pick a China model where the government decides everything that should happen.”