The Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) Study was conceived for the first time in 1999. A group of European researchers, mainly nutritionists and paediatricians prepared a first application for the European Union 5th Framework Programme. At that time, the group was lead by Professor Ascensión Marcos, and she had the important task of promoting the core study idea and to motivate us in this long-term project. From the beginning, one of the key ideas was to try to follow the highest quality standards.

Some of the key researchers involved in the current HELENA Study were already part of the first application. Professor Marcela González-Gross, Dr Mathilde Kersting, Professor Frédéric Gottrand and Professor Denes Molnár were working on the project from the beginning, and constitute key elements of the present research group.

After working for several years to improve the project, we successfully obtained the financial support of the European Commission in the second call of the 6th Framework programme. During these years, some colleagues and friends left the group, and I take this opportunity to acknowledge them for their contributions.

The decision to submit the last application was made in Rome during the Congress of the European Federation of Nutrition Societies, which was held in October 2003. I thank the persons present in that meeting for supporting me to become the new HELENA coordinator. I considered this nomination as a heavy responsibility and the major professional challenge in my academic career. During the period of application preparation I felt really supported by the group.

The first objective of the HELENA Study is to describe the nutritional status of the European adolescent population. To deal with this objective and to cover a broad European area, some new research groups entered our consortium. To deal with the European Commission call, we also incorporated some groups and small and medium enterprises, to contribute with their knowledge in consumer behaviour, sensory analysis and food development. Their contribution has also been a determinant of the success of the application and the adequate development of the project.

A basic objective of the HELENA Study is to develop valid and innovative methods to assess the different dimensions of nutritional status in the adolescent population. This supplement includes 10 papers. Nine are concerned with the HELENA Cross-Sectional Study, and one with the HELENA lifestyle education intervention. All of them consider different methodological issues that provide insights into the validity and reliability of the tools we are using in our study. One practical goal of the HELENA Study is to provide other European and International researchers with these tools. Therefore, it seemed important to know their main performances when applied to the assessment of adolescents’ nutritional status.

To guarantee the quality of the Study, we selected three outstanding researchers as our external advisors, by alphabetical order: Dr Francesco Branca, Professor Barry Popkin and Professor Artemis Simopoulos. I take this opportunity, in the name of the HELENA consortium, to thank them for their important contribution to the study. They also kindly accepted to be our guest editors in this supplement of the International Journal of Obesity. They were the main participants in the internal peer-review process that was conducted prior to the submission to the journal. This editorial step was a key element to ensure the quality of the manuscripts to be included in the supplement, because their comments strongly improved the preliminary versions.

In the name of the HELENA consortium, I wish you pleasant reading of the supplement papers. I also hope that they will be relevant for the researchers involved in the assessment of the nutritional status of the adolescents all over the world.