The Anatomical Therapeutic Chemical Classification System is used for the classification of drugs. It is controlled by the WHO Collaborating Centre for Drug Statistics Methodology, and was first published in 1976.

Drugs are divided into different groups according to the organ or system on which they act and/or their therapeutic and chemical characteristics.

In the system drugs are classified into groups at 5 different levels:

1st level: The first level of the code is based on a letter for the anatomical group, there are 14 main groups. (one alpha character)

A	Alimentary tract and metabolism
B	Blood and blood forming organs
C	Cardiovascular system
D	Dermatologicals
G	Genito urinary system and sex hormones
H	systemic horomal preparations, excluding sex hormones and insulins
J	Anti-infectives for systemic use
L	Antineoplastic and immuomodulating agents
M	Musculo-skeletal system
N	Nervous system
P	Antiparasitic products, insecticides and repellents
R	Respiratory system
S	Sensory organs
V	Various

2nd level: therapeutic main group. (two numeric characters)

3rd level: therapeutic/pharmacological subgroup. (one alpha character)

4th level: chemical/therapeutic/pharmacological subgroup. (one alpha character)

5th level: subgroup for chemical substance. (two numeric characters)

The actual drug name used is the international non-proprietary name (INN).

The ATC/DDD system is the ATC system with the addition of a measure of the assumed average maintenance dose per day for a drug used for its main indication in adults (Defined Daily Doses).

Other ATC classifications are ATCvet (for veterinary medicinal products) and ATC herbal classification (for herbal remedies).

The ATC classification system was based on the Anatomical Classification (AC-system) developed by the European Pharmaceutical Market Research Association (EPhMRA) and the Pharmaceutical Business Intelligence and Research Group (PBIRG).