In reference to anatomy, the smallest level of organizational classification used when studying the structure of a living organism.

The organ system level deals with the interrelationship between groups of two or more organs that together perform one or more functions, and their interaction with the body of the organism that they comprise.

Due to the complexity of a living organism, it may be studied on one or more of several structural levels. These levels are as follows, starting from smallest (micro) to largest (macro).

1. Chemical Level
2. Organelle Level
3. Cellular Level
4. Tissue Level
5. Organ Level
6. Organ System Level
7. Organism Level