Refers to a relationship between tables in a database when a parent row in one table contains several child rows in the second table, and vice versa. Many-to-many relationships are often tricky to represent. However, one or more rows in a table can be related to 0, 1, or many rows in another table. In a many-to-many relationship between Table A and Table B, each row in Table A is linked to 0, 1, or many rows in Table B and vice versa. A 3rd table called a mapping table is required in order to implement such a relationship.