Chronic pain is defined as one that lasts for at least three to six months, provided that this time is greater than the normal healing period of an injury. This pathology has become an important public health problem due to its high prevalence. In particular, it is estimated that it affects 20-30% of the adult population around the world. Medical interventions are the first-line treatment in recent clinical practice guidelines. Unfortunately, the effectiveness of medical interventions is only modest. Treatments significantly reduce pain on average, but the effect tends to be small . In addition, the drugs are not effective for a large percentage of patients. What these and other research suggest is that focusing only on large sample studies and the use of average change scores calls into question the usefulness of current patient-centered treatments. As noted by Dr. Turk, when data are averaged, various pain syndromes, drugs, surgical procedures, and studies in different countries are often included, which may mask the efficacy results of different treatments with Different samples. Single case methodology could be one of the ways to overcome these limitations. The single case investigation is a type of experimental study that offers experimental control within a single case. Some studies have already demonstrated the usefulness of these designs in chronic pain. In fact, the benefits of using this methodology as opposed to large sample studies were discussed recently during the 10th Congress of the European Pain Federation. These benefits include the need for a reduced number of participants, the ability to follow clinical evolution in real time and continuously, the amount of data provided, and applicability when using a control group is impractical or unethical. The investigators conducting the present investigation recently conducted a study at the Pain Unit of the Hospital General de Castellón to explore the effectiveness of current medical treatments. According to previous investigations, the treatment effect was only small (d = 0.32) and only a percentage of the patients (18.1%) had a clinically significant reduction of pain (ie a reduction greater than 30% ). From these results and the literature reviewed, a single case design could be an alternative method for research in the Pain Unit. However, the implementation of this type of methodology can be very costly due to the continuous evaluation of the evolution of the patient. In this sense, several studies have already shown that mobile applications (app) can effectively control the evolution of a wide range of pathologies in health settings . In fact, a recent controlled clinical study found that ecological records performed with mobile app had greater reliability than paper-and-pencil records. Rosser and Eccleston conducted a comprehensive review of existing apps for pain and the conclusion was that there was little evidence to support the use of current apps. Specifically, these authors showed that most apps did not specify whether their content was validated, did not include psychological components and none had been applied in a clinical study. In light of these results, Dr. Carlos Suso Ribera and Dr. Ribera Canudas contacted the research team Labpsitec of Jaume I University, who were developing an application for pain called "Pain Monitor". Dr. Carlos Suso Ribera and Dr. Ribera Canudas of the Pain Unit collaborated with Dr. Azucena García Palacios and Dr. Diana Castilla López of the Universitat Jaume I in the final development of the application of pain, Following the guidelines of Rosser and Eccleston. App content validity and usability was tested in a previous study at the pain unit of the Vall d'Hebron Hospital (Suso-Ribera et al., in preparation). Thus, the study objective is to test if the introduction of this tool in the day-to-day work of the pain unit improves pain management of chronic pain patients. To do this, it is necessary to compare the evolution of patients who follow the usual treatment at the pain unit (without app) with a group of patients who do use the Pain Monitor app.