Constructed wetlands (CWs) are artificial wastewater treatment systems appropriate for small communities because of their affordability, operability and reliability. These qualities are true whenever CWs are designed and constructed properly, and as long as the necessary operation and maintenance procedures are carried out correctly. Experience shows that the operation and maintenance procedures, and the frequencies with which these procedures are carried out, differ from one CW to another. With this in mind, and along with a projected increase in CWs in Catalonia, the Catalan Water Agency (Agència Catalana de l'Aigua) has developed an Environmental Decision Support System (EDSS) which proposes guidelines for monitoring and maintenance, according to the characteristics of each CW. This EDSS was developed following a methodology based on five steps: (i) problem analysis; (ii) collecting data and knowledge acquisition; (iii) model selection; (iv) model implementation and (v) validation. This paper describes the methodology followed to build the decision support system and presents some examples of the information provided by this EDSS.