The affiliations between the majors and minors are contracts that are drawn up to last anywhere from one to five years. The salaries of the minor league players are paid by the major league teams. All other costs and expenses are taken care of by the minors.

Affiliates signed by the major league clubs do change, due to financial and competitive changes, or perhaps due to a move. For instance, the Houston Astros, in an effort to improve its fan base to a wider area of Texas, switched their affiliation from the Zephyrs to Round Rock. The Zephyrs were forced to find another team to sign with in order to remain in the system of affiliation. Currently, the longest lasting affiliation has been between the Orioles and their Rookie-level Appalachian League, the Bluefield Orioles, which has been around since 1958.

An affectionate term for the minor leagues is the