Federal law allows recording of phone calls and other electronic communications with the consent of at least one party to the call. A majority of the states and territories have adopted wiretapping statutes based on the federal law, although most also have extended the law to cover in-person conversations. Thirty-eight states and the District of Columbia permit individuals to record conversations to which they are a party without informing the other parties that they are doing so. These laws are referred to as "one-party consent" statutes, and as long as you are a party to the conversation, it is legal for you to record it. (Nevada also has a one-party consent statute, but the state Supreme Court has interpreted it as an all-party rule.)
Twelve states require, under most circumstances, the consent of all parties to a conversation. Those jurisdictions are California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania and Washington. Be aware that you will sometimes hear these referred to inaccurately as "two-party consent" laws. If there are more than two people involved in the conversation, all must consent to the taping.