The Green Mystery Guest Checklist is a tool that can be used by Quality Inspectors, Mystery Guests, Quality Managers in the tourism and accommodation sector to assess or self-assess the sustainability indicators of the different areas involved in the accommodation ecosystem (food, service, supplies, transport, events, external communication and methods of encouraging sustainable attitudes and behaviors towards guests, etc.). The tool will be produced in English and translated into the languages of the participating countries (Italian, Spanish and Romanian) to ensure its widespread use in the national territories and beyond. The Green Mystery Guest Checklist is based on a series of 100 sustainability elements relating to all the relevant areas for accommodation that may be present or absent and are managed by the Mystery Guests. Given its ease of use, this tool can also be used as an internal reference to monitor all aspects related to sustainability in an accommodation facility and possible areas for improvement. The evaluation sheets for the sustainability elements will be completely digitalized and published on the official project website and able to produce an automated report with a score in hundredths. The selection of sustainability indicators will be based on the following methods of analysis and research: 1. analysis and review of scientific research, laws and directives at national and European level in the different sectors of reference (food, services, supplies, transport, events, external communication, etc.) 2. field research through 24 semi-structured interviews determined jointly and addressed to experts in the fields analyzed in the different sectors of reference (food, services, supplies, transport, events, external communication, etc.)