Shortly after 1934 when Kögl and co-workers found that indole–3–acetic acid is a substance which has cell elongation activity in plants, many attempts were made to find other chemicals showing an indole–acetic–acid-like activity. It is 25 years now since Zimmermann and Wilcoxon found that naphthyl–1–acetic acid and some other aryl-acetic acids are active plant regulating compounds also. With these experiments it could be shown that the indole-nucleus is not essential for a plant growth regulator. The findings of Irvine showed that 2-naphthoxy-acetic acid is equally active as a phytohormone. This indicates that the acetic acid has not necessarily to be linked to the naphthalene ring directly. Thenaphthoxy-acetic acid was further modified chemically and became the starting point for the most widely used phytohormones: 2,4-D, MCPA and 2,4,5-T.