This is a system and user guide to MECODA, an online service that provides a repository of tools that facilitate analysis and visualisation of citizen science data from citizen observatories (COs). MECODA (ModulE for Citizen Observatory Data Analysis) is one of the technological services developed in the Cos4Cloud project framework by Instituto de Ciencias del Mar (ICM-CSIC). More information:https://cos4cloud-eosc.eu/services/mecoda-data-analysis-package/ This system and user guide is one of the Cos4Cloud Training and Capacity Building Resourcesdeveloped by The Open University (OU), in collaboration with project partners, forthe Cos4Cloud Toolbox & Evidence Hub. System and user guide designer: Harriett Cornish, OU. Contact: cos4cloud-toolbox@open.ac.uk