Research software (RS) is designed to facilitate answering research questions, and it achieves this goal by running calculations, modeling some phenomena or managing and visualizing datasets, among many others. This is what RS is meant to accomplish, first and foremost, and everything that does not go in this direction is never a priority. GUIs in research software are nearly always considered as an aftermath - if considered at all! -, and add-on usually not very sophisticated to try to make the software more accessible. And the result is rarely fit for purpose. In this first lecture of the course we will try to answer the question of when research software might benefit from a GUI, when it will not, and what the benefits that having a well design GUI can bring to the table. This presentation is part of the workshop "Graphical User Interfaces for Research Software", delivered by Diego Alonso Álvarez and supported by the Software Sustainability Institute and Imperial College London.