When to use & instead of "and"

Are there rules of usage when using the ampersand "&" instead of "and"?

Are they completely interchangeable?

The ampersand seems more casual, but I'm not sure.

Solution 1:

There are very, very few acceptable uses of & in proper written English. Here are some of them:

& is especially common when joining names to indicate a firm or a partnership, for example, a law firm:

Baker & McKenzie
Abercrombie & Fitch
Crosby Stills Nash & Young

In abbreviations, when abbreviating "and", & is often used:

AT&T (American Telephone and Telegraph)
P&L (profit and loss)
R&D (research and development)

One rare usage is on envelopes addressed to a couple:

Mr. & Mrs. Jackson

&c. is a rare and somewhat archaic looking abbreviation for etc.

Other than that it is vanishingly rare to see & in formal written English, although of course in informal email, text messages, notes, and handwriting, anything goes.

Solution 2:

Are there rules of usage when using the ampersand "&" instead of "and"?

I looked through a couple of reference books and both of them said that the ampersand should only be used in company names.

Are they completely interchangeable?

Meaning-wise I think they are.

The ampersand seems more casual, but I'm not sure.

It's an abbreviation so one might use it more in less formal writing.