An evaluation of small wastewater treatment plants (WWTPs) treating the effluents of communities with <2000 inhabitants in Catalonia (north-east of Spain) was carried out to establish the definition of appropriate wastewater treatment (AWT) and the criteria for the selection of the alternative processes that can achieve the AWT. The implementation of an AWT in these communities before the end of the year 2005 is necessary to comply with the European Directive 91/271. The AWT is that which permits the accomplishment of quality objectives in receiving waters after the discharge of the effluents. The standards for the AWT in each particular case are set out through a contaminant loading balance. Nevertheless, the final applied standards can only be as strict as they are for larger WWTPs. These criteria are very close to those stated in the French legislation, and it is felt that they are at the same time environment-friendly and realistic for small WWTPs. Secondary treatments are generally recommended to achieve the AWT. Natural wastewater treatment systems are preferred over conventional treatments to achieve the appropriate treatment because they are simple to operate, can reach the same level of treatment efficiency and have lower operation and maintenance costs. The policy debate that has produced all these statements is described.