Whenever you invest, it is critical that you understand the terms of your investment and analyze the risks it may pose.

Typically, much of this detailed information is contained in a "prospectus." You don't want to rely exclusively on your broker or financial adviser to know what is in the prospectus.

But unfortunately, even getting a prospectus can be difficult.

At least, that's the message from one reader, who said his broker was not required to provide him with a prospectus on auction-rate securities. It was up to him to track down the document, on file at www.sec.gov.

Indeed, Securities and Exchange Commission spokesman John Heine confirms that not everyone must get a prospectus when they invest. But you certainly should ask for documentation on your investment and perhaps, conduct your own research. Much is available to search for free at www.sec.gov.

Take mutual funds. A prospectus must be delivered along with a confirmation the first time you buy a mutual fund. Then, the general practice is for the fund group to send you an updated prospectus whenever it's updated.

Most exchange-traded funds abide by this same general rule, Mr. Heine says. But certain types of exchange-traded funds may opt instead to provide you with a "product description," which is different. "Because of the fact that it's fairly difficult distinction to make, the general practice is (for exchange-traded funds) to provide the prospectus," Mr. Heine says.

If you're investing in an initial public offering or a "follow on" offering of stock, you're required to get a prospectus. You're also required to get a prospectus for bonds, offered in a "registered offering."

But don't expect a prospectus to come with bonds issued by the U.S. government, which does not register its securities with the SEC.

Meanwhile, say you direct your broker to buy 100 shares of General Motors. Don't expect a prospectus. That's because you're buying the stock on a "secondary market." You'll have to do your own research.

Investing in a CD or other deposit? Federal Truth in Savings rules may apply. These rules, according to the Federal Reserve, require banks to issue disclosures to consumers. But they do not generally require banks to prepare disclosures for deposits sold through brokers. Meanwhile, the SEC does not generally regulate bank deposits.

But Truth in Savings advertising rules do apply to brokers offering deposits. A broker's written or oral ads, which may invite, offer or announce a deposit, are prohibited from misleading consumers.

And for municipal bonds, says Larry Lawrence, policy and technology adviser for the Municipal Securities Rulemaking Board, Alexandria, Va., you must get an "official statement" during the first 25 days after the bond is issued. Municipal fund securities, which include college savings 529 plans, for example, must provide an official statement for the first sale.

An official statement is equivalent to a prospectus, Mr. Lawrence says. You also can find ongoing updated information on municipal bonds and municipal securities at www.emma.msrb.org.

Do you have an income exceeding $200,000 in each of the two most recent years or a net worth that exceeds $1 million? If so, you may be eligible for certain investments that are exempt from SEC registration. So federal laws may not require that you get documentation. But nevertheless, you certainly should ask for it. You might get a "private placement memorandum." However, unlike a prospectus, a private placement memorandum is not regulated by the SEC.

Beware that there is an increasing move to offer disclosure documents online.

Once you access a prospectus online, the obligation to distribute the document typically is fulfilled, Mr. Heine says. So be sure to read it carefully.