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The Once-Only Hub.Collaborate to build the future of the Single Market.

The Once-Only Technical System connects EU public authorities, so they can exchange official documents and data at the citizen’s request. It puts into practice the Once-Only Principle, which states that citizens should not be forced to provide information to authorities if another authority already holds that information in electronic format.

Who is the Once-Only Hub for?

The Hub is primarily for implementers from the EU Member States, building and maintaining the Once-Only Technical System. It is a single environment providing reliable information, services and support. Anyone can browse the Hub to learn more about the system, programme and the key benefits for EU citizens and businesses.

Introduction to the Once-Only Technical System (OOTS)

The Once-Only Technical System is a technical framework for data sharing. As of the end of 2023, citizens and businesses will be able to request official documents (known as “evidence”) to complete administrative procedures allowing people to study, work, live and retire in other EU countries in 7 simple steps. The decentralised architecture connects public authorities, so they can exchange evidence at the citizen's or business' request.

Discover the 21 procedures covered by the SDG Regulation in the scope of Once-Only Technical System and how EU citizens and businesses can benefit from Once-Only Technical System.

Discover the Once-Only Technical System

Once-Only Technical System - user journey for cross-border procedures

The user-journey refers to the steps anyone, a citizen or business, must take when using the Once-Only Technical System with a mind to the overall experience the user has. The Once-Only Technical System aims to make the experience of studying, working and living in other EU Member States as simple as possible.

  • 1

    Authenticate: The user authenticates with an eIDAS notified eID scheme

  • 2

    Locate evidence: The procedure portal identifies what to ask for (Evidence Type), and who to ask it from (Evidence Provider)

  • 3

    Evidence request: The request for evidence is sent to the Evidence Provider

  • 4

    Redirect: The user is redirected to the provider side

  • 5

    Preview: The user previews their evidence

  • 6

    Evidence response: The Evidence Provider responds to the evidence request

  • 7

    Submit: The user completes and submits his/her procedural form

Visit the Once-Only Technical System user journey page for a detailed overview of the 7 steps in the Once-Only Technical System (OOTS) journey.

Discover the OOTS user journey

Mapping of architecture to the user journey

"The Once-Only Technical System is a distributed collection of systems. Most of these are independently established and maintained by different EU Member States and, once interacting with one another, form a Once-Only technical "ecosystem."

The Once-Only Technical System architecture adopts a decentralised approach based on integration and interconnection of independent systems by using a loosely coupled ‘interoperability layer’ based on the concept of common, reusable "Building Blocks".

Understand how the Once-Only Technical System's decentralised high-level architecture connects public authorities so that they can exchange evidence at the request of a citizen or business."

Learn about the OOTS architecture


OOTS Guidance Presentation

The Guidance Presentation provides a useful introduction for teams across the EU working on the implementation of the Once-Only Technical System.

Download OOTS Guidance Presentation

Common and supporting services

To provide comprehensive support and proper exchange of evidence between Data Services and Online Procedure Portals, the Once-Only Technical System uses Once-Only common services and additional supporting services.

Go to common & supporting services

Technical design

The technical and operational specifications of the Once-Only Technical System, are complemented by detailed, non-binding technical design documents jointly prepared by the Commission and Member States.

Go to Technical design documentation

Collaborate with us

SDG Once-Only Collaborative Space

In the context of the Once-Only Technical System, the mission of this group is to coordinate the implementation work around the setup of the Once-Only Technical System. The members of this group are the Member States (and other public entities). This groups is only accessible by representatives of the Member States (and other public entities).

Restricted space


Six sub-groups have been set-up to examine specific topics related to the Once-Only Technical System and are expected to report to the Single Digital Gateway Coordination Group. They enable the participation of all Member States in the discussion and ensure the work done by Member State experts is monitored. The sub-groups can only be joined by experts officially nominated by their National Coordinators.

Restricted space


OOTS Projectathon collaborative space

The objective of this space is to encourage dialogue and exchange of ideas between stakeholders in an open and transparent environment. It will mainly serve to promote the Implementers Café series in preparation of the Projectathons. Space users should be implementers of the Once-Only Technical System.

Service desk

The Service Desk helps users improve the performance, continuity and reliability of their Once-Only Technical System implementations.

Contact the Service Desk

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