Unfortunately, there's no easy answer to this question, and a mistake can land you in jail. Illegal tape recording can have both criminal and civil penalties. My advice is almost always: When in doubt, don't.
Still, many employees want to record a boss or HR at work, and there are good reasons to do so. If you have a sexual harasser, it's handy to catch them red-handed. It's hard to deny something a judge or jury can hear in the harasser's own voice. Some employees want to record meetings with HR to make sure they get all the important information or to have evidence of the reason given for termination or discipline. Other employees want to get evidence of discrimination or other illegal practices of the employer. Sadly, while you can go to jail if you illegally record a conversation, even of a bigot, there is no law making workplace discrimination a crime in the U.S.
Here's what you need to know about recording conversations at work:
All-party consent: Eleven states, California, Connecticut, Florida, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania, and Washington, require all parties to the conversation to consent to being taped. Illinois' all-party consent law was found to be unconstitutional. Hawaii, a one-party consent state, requires all-party consent if the device is installed in a private place. Massachusetts bans "secret" recordings. These laws are sometimes referred to as "two-party consent" laws, but if there are three people in the conversation, all three must consent. The Digital Media Law Project has a handy state-by-state resource here. The Reporter's Committee for Freedom of the Press has another detailed state-by-state guide here.
Expectation of privacy: You can almost always record conversations in public areas, because the courts say there's no "expectation of privacy" in those places. Whether or not you are a party to the conversation, if it's out there in public, you may be allowed to tape it. Here's where it gets tricky. Many courts have held that there's little or no expectation of privacy in the workplace. There are cases saying, for instance, that a party to a conference call has no expectation of privacy. If you're in a group meeting at work, is there an expectation of privacy? Possibly not.
As an example, cases in my home state of Florida on the expectation of privacy at work say things like: "Society does not recognize an absolute right of privacy in a party's office or place of business." "[A]lthough defendant may have had reasonable expectation of privacy in his private office, that expectation was not one which society was willing to accept as reasonable or willing to protect." "Society is willing to recognize a reasonable expectation of privacy in conversations conducted in a private home. However, this recognition does not necessarily extend to conversations conducted in a business office."
The problem I have with relying on cases like these to tape at work is the use of weasel-words like "necessarily" and "absolute" and "reasonable." These cases are very fact-specific and that means a court could still find that your boss or coworker had an expectation of privacy. If you get it wrong, you can end up in jail.
Retaliation: If you record a conversation to document illegal discrimination or illegal harassment (we're talking harassment or discrimination based on race, age, sex, religion, national origin, disability, pregnancy, or other protected category, not bullying), then you may or may not be protected against retaliation by your employer. The courts have split on this issue. Depending on your state, your employer may be allowed to fire you for recording a conversation at work.
To summarize, you can probably tape a conversation at work that you're part of as long as you live in one of the 39 one-party consent states. You can also possibly tape a conversation that's in a public area (lobby, office or conference room with doors open, stairwell). You can maybe tape a conversation in the office behind closed doors. Only 35 states allow recording of phone calls with one-party consent. If you get it wrong, you're in big trouble, so be careful.
My best recommendation in all-party consent states continues to be, when in doubt, pull out your recorder and turn it on. Say, on the recording, "You don't mind if I tape this do you?" If the other person or people say they don't mind, keep recording. If anyone objects, turn it off. Pull out a pad of paper and a pen and take good notes instead. No potential case against your employer is worth risking jail time.