Ludwig van Beethoven's Opus 18, published in 1801 by T. Mollo et Comp in Vienna, consisted of his first six string quartets. They were composed between 1798 and 1800. The order of publication (numbering within the opus) does not correspond to the order of composition. See:

• String Quartet No. 1 in F Major
• String Quartet No. 2 in G Major
• String Quartet No. 3 in D Major
• String Quartet No. 4 in C Minor
• String Quartet No. 5 in A Major
• String Quartet No. 6 in B flat Major

See also: List of works by Beethoven