The wastewater purification process is influenced by the presence of microorganisms that degrade and convert organic matter, generating food for other organisms that intervene in the purification process. Among these organisms we find bacteria, fungi, protozoa, metazoans, each one playing an important role in the purification process. Its study is decisive when evaluating the efficiency of the process. Each type of organism is an indicator of the circumstances that arise in the reactor. Based on this, biotic indices have been developed that advise us on the status of an active sludge system and that allow an improvement in the exploitation of purification facilities.