The Delphi Technique (DT) is a research methodology that uses turns or rounds between the researcher and a group of experts, until a desired level of agreement or consensus between the members of the expert panel has been reached with regards to the question in hand. Despite its popular use in the social sciences' domain and its progressive adoption by nurse-researchers internationally, in Greece, there is a lack of its application; its comprehension, usage, and methodological merits are still in question within the Greek nursing research community. Aim: To provide a detailed presentation of the epistemological and practical value of the DT, through a series of four consecutive papers which expand on the following: i) the classical methodological approach and its main modifications, ii) the methodological reflections iii) analysis and presentation of results, and iv) the use and usefulness of the method in application to nursing research. The first of the four papers provides a description and a comparative assessment of the main variations of the DT. Results: A review of the literature elucidated three main sub-types of technique: the Classic or Conventional Delphi, the Policy Delphi and the Decision Delphi Technique. Nevertheless, due to many variations of the basic philosophy of the method and the many differentiations in its application, it is quite difficult to classify each study that used Delphi, with precision. The main methodological differences that can challenge the researcher are discussed and finally, examples that show the advantages and the usefulness of the individual Delphi techniques are given. Conclusions: Use of the DT in research has many advantages and it can become exceptionally useful once the content of the method and its practical applications are fully understood.