Minnesota law requires that an employer must put an offer of employment in “a written and signed agreement of hire,” such as an offer letter.  The document must include:

  1. The date on which the agreement was entered into;
  2. The date on which the services of the employee are to begin;
  3. The rate of pay per unit of time, or of commission, or by the piece, so that wages due may be readily computed;
  4. The number of hours a day which shall constitute a regular day’s work, and whether or not additional hours the employee is required to work shall constitute overtime and be paid for, and, if so, the rate of pay for overtime work; and
  5. A statement of any special responsibility undertaken by the employee, not forbidden by law, which, if not properly performed by the employee, will entitle the employer to make deductions from the wages of the employee, and the terms upon which such deductions may be made.

See Minn. Stat. § 181.55.  This requirement applies to any work to be performed in Minnesota and to any work to be performed in another state for an employer that is localized in Minnesota.
Unless the employment is not intended to be at will, the letter should expressly state that the employment is at will.  For at-will employment, employers should avoid words or phrases that might indicate a contract or promise of employment for a definite period of time, or that might indicate that the employee may only be terminated for “just cause.”
The offer letter should also address whether the employment is contingent upon fulfillment of any outstanding requirements, such as receipt of favorable references (if references have not been checked), successfully passing a pre-employment drug test or background check, or fulfillment of any other permissible pre-employment conditions.