The letter came in a plain gray envelope. I had been turned down for a personal loan, it said, due to information Innovis and SageStream had acquired about me.

I vaguely knew that Innovis was a mini credit bureau specializing in fraud detection. But what was SageStream? I decided to find out. Turns out, it’s just one of dozens of obscure agencies collecting data about me, you, and everyone we know—information like how quickly we pay the electricity bill, which medicines we’re taking, or whether we’re up-to-date on child support payments.

When people think of credit-reporting agencies, they usually think of Equifax, Experian, and TransUnion. But credit bureaus like the big three are largely limited to information about financial services, like loans and credit cards, says John Ulzheimer, a credit expert. A universe of other, secret companies, often referred to as data brokers, hoover up and sell our personal information. They can make or break your ability to get a job, obtain health or life insurance, rent an apartment, cash a check, or even gamble at a casino.

And it’s a safe bet you’ve never heard of most of them.

According to the Consumer Financial Protection Bureau, there are 47 major consumer reporting agencies across nearly a dozen categories. They in turn exchange your personal data with thousands of smaller information brokers across the Internet. Here are the ones you should know about.

Consumer credit agencies

The big three know more about you than Santa Claus does. Whenever you apply for a credit card, take out a loan, are late paying your mortgage, get harassed by a debt collector, file for bankruptcy, or have a lien placed against your property, these agencies maintain a record of it.

They also maintain a record of your past addresses, names, businesses, and employers, along with at least part of your Social Security number. (This is why the Equifax hack, through which nearly 150 million people had this information stolen, was such a catastrophic blunder.)

The good news is that you can request a free copy of your report from each agency every year, as well as whenever a report results in an “adverse action” like being turned down for a loan. (Just be sure to make the request via AnnualCreditReport.com; otherwise you could be scammed by a lookalike site that charges you money for this service.)

You can also ask the bureaus to correct errors, and that’s important. A 2015 FTC survey found that one in five credit reports contains “material errors” that could impact your credit score. So be sure to read them carefully.

Supplementary agencies

These mini credit bureaus collect information the big three often don’t, like property ownership, licensing information, child support payments, or your billing history with wireless carriers and Internet service providers. The main mini bureaus include CoreLogic CredCo, Innovis, and LexisNexis. Others, like SageStream, offer proprietary (and inscrutable) credit ratings designed for loan providers to weigh alongside an applicant’s traditional FICO scores.

Employment screeners

More than 100 companies provide background checks for potential employers, which may include arrest and criminal records, as well as worker’s compensation claims. There are about a dozen major employment screeners, including Sterling Talent Solutions, Intellicorp (owned by Verisk Analytics), and HireRight.

The good news is that an employer has to obtain your consent before it can request a background screening, and it must identify the reporting agency it uses. If the company decides to turn you down based on that screening, it must send you a “pre-adverse action letter” that allows you to review the background check, correct any inaccuracies, and ask to be reconsidered.   

Tenant screeners

If you’ve ever been late with your rent or evicted from your apartment, agencies like Experian RentBureau, CoreLogic Rental Property Solutions, or Contemporary Information Corp. (CIC) could rat you out to a potential landlords.

Personal-property insurance screeners

Agencies like LexisNexis’ C.L.U.E., Insurance Information Exchange (owned by Verisk), and Drivers History (owned largely by TransUnion) record every claim you’ve filed on your home or automobile, and/or every moving violation, over the past seven years. They share this data with insurance companies, which use it to help determine whether you qualify for insurance—and, if so, how much you’d have to pay. Some share this information with employment screeners.

Subprime loan and bank screeners

Firms like Clarity Services, DataX, and FactorTrust collect information on payday and installment loans, check-cashing stores, and other financial services aimed at low-income consumers. If you’ve been bouncing checks (or someone is writing bad checks in your name), companies such as ChexSystems, TeleCheck, and Early Warning Services share this information with banks.

Miscellaneous data brokers

There are lots more. The National Consumer Telecom & Utilities Exchange is probably the largest consumer reporting bureau no one has heard of, Ulzheimer says. If you fail to pay your water, gas, electricity, cable, or phone bill on time, the NCTUE knows.

Ever been turned down for health or life insurance because of a pre-existing condition? You can probably thank the Medical Information Bureau, which shares medical-claim data among its more than 400 members. (And given the precarious state of Obamacare, you may have to worry about this again soon.) Milliman IntelliScript, meanwhile, records every prescription drug you’ve ever purchased via your insurer.

The Retail Equation tracks the products you return to stores and flags transactions it thinks are fraudulent. If stores are refusing to issue you refunds, you need to request a copy of your Return Activity Report.

If you’ve crapped out too many times at the gaming tables, data collected by Certegy Check Services may determine if the casino honors your marker or tosses you out on your ear.

Under the Fair Credit Reporting Act of 1970, all consumer reporting agencies are required to send you a free report, upon your request, but none are required to make it easy for you, Ulzheimer notes.

At AnnualCreditReport.com you can fill out an online form for any of the big three credit-reporting agencies. For the rest, you typically submit an application by mail or fax, along with proof of identity and your address; some also accept requests via a torturous interactive voice system.

Unfortunately, despite grossly negligent behavior by companies like Equifax, you can’t stop these agencies from collecting information. So the best strategy is to keep a watchful eye on them by requesting your reports and correcting bad information.

“There is no federal or state law you can leverage to make it illegal for any of the credit-reporting agencies to maintain a file on you,” Ulzheimer says. “But trying to get a loan without a credit report would be like trying to get a job without having anything on your résumé. Having an accurate credit report is infinitely better than having no credit report at all.”