Unwanted mental intrusions (UMIs) and their functional consequences had been proposed as symptom dimensions in current cognitive models of Obsessive-Compulsive Disorder (OCD), Body Dysmorphic Disorder (BDD), Illness Anxiety/Hypochondriasis (IA/H) and Eating Disorders (EDs). The main purpose of this doctoral dissertation was to ascertain the transdiagnostic nature of UMIs, above and beyond their specific contents, i.e., obsessional, appearance defects, illness and death, and eating disorders-related. To this end, the Questionnaire of Unpleasant Intrusive Thoughts (QUIT) was designed, and, based on it, four studies were conducted. Studies 1 and 2 examined the transdiagnostic nature of UMIs, based on their respective frequencies and disturbances and the similarities and differences in the functional links among the four UMIs that participants selected as the most upsetting. A non-clinical Spanish population (N=438) and patients with a diagnosis of OCD (N=39) participated in these studies. Study 3 searched for the putative etiopathogenic role of UMIs and examined whether individuals with OCD and those who are vulnerable to the disorder experience more UMIs than people who are not vulnerable to OCD. In this study subjects without risk of OCD (n=68), at risk of OCD (n=71), and OCD patients (n=39) participated. Study 4 examined the transdiagnostic nature of UMIs across different countries and socio-cultural contexts. The objectives were to analyze the putative cultural differences that might influence the universality of UMIs and their form across cultures, as well as their hypothesized transdiagnostic value. Non-clinical participants (N=1473) from seven countries in the Middle East, Europe, and South America participated in this study. Results support the transdiagnostic nature of unwanted mental intrusions in the general population from different countries and cultural contexts, as well as in patients with OCD, revealed by: a) the high percentage rates of participants who experienced the four types of UMIs; b) the relationships among the UMIs in terms of their frequency and the disturbance they provoked; and c) the similarities in the frequency and disturbance due to the UMIs and in the functional consequences of the most disturbing UMIs. The etiopathogenic role of intrusions was supported by: a) the associations between the UMIs and their respective clinical measures; b) the predictive power of intrusions to explain scores on symptoms of these disorders; and c) the increasing tendency to experience all UMIs among individual at risk for OCD, compared to individuals with no risk of OCD. To conclude, the transdiagnostic nature of UMIs across OCD, BDD, IA/H and EDs could help to explain the phenomenological similarities and comorbidities found among these disorders, supporting the inclusion of IA/H and BDD in OC-spectrum disorders. The QUIT can be reliably used as a screening instrument to detect vulnerable individuals in different contexts and cultures.Unwanted mental intrusions (UMIs) and their functional consequences had been proposed as symptom dimensions in current cognitive models of Obsessive-Compulsive Disorder (OCD), Body Dysmorphic Disorder (BDD), Illness Anxiety/Hypochondriasis (IA/H) and Eating Disorders (EDs). The main purpose of this doctoral dissertation was to ascertain the transdiagnostic nature of UMIs, above and beyond their specific contents, i.e., obsessional, appearance defects, illness and death, and eating disorders-related. To this end, the Questionnaire of Unpleasant Intrusive Thoughts (QUIT) was designed, and, based on it, four studies were conducted. Studies 1 and 2 examined the transdiagnostic nature of UMIs, based on their respective frequencies and disturbances and the similarities and differences in the functional links among the four UMIs that participants selected as the most upsetting. A non-clinical Spanish population (N=438) and patients with a diagnosis of OCD (N=39) participated in these studies. Study 3 searched for the putative etiopathogenic role of UMIs and examined whether individuals with OCD and those who are vulnerable to the disorder experience more UMIs than people who are not vulnerable to OCD. In this study subjects without risk of OCD (n=68), at risk of OCD (n=71), and OCD patients (n=39) participated. Study 4 examined the transdiagnostic nature of UMIs across different countries and socio-cultural contexts. The objectives were to analyze the putative cultural differences that might influence the universality of UMIs and their form across cultures, as well as their hypothesized transdiagnostic value. Non-clinical participants (N=1473) from seven countries in the Middle East, Europe, and South America participated in this study. Results support the transdiagnostic nature of unwanted mental intrusions in the general population from different countries and cultural contexts, as well as in patients with OCD, revealed by: a) the high percentage rates of participants who experienced the four types of UMIs; b) the relationships among the UMIs in terms of their frequency and the disturbance they provoked; and c) the similarities in the frequency and disturbance due to the UMIs and in the functional consequences of the most disturbing UMIs. The etiopathogenic role of intrusions was supported by: a) the associations between the UMIs and their respective clinical measures; b) the predictive power of intrusions to explain scores on symptoms of these disorders; and c) the increasing tendency to experience all UMIs among individual at risk for OCD, compared to individuals with no risk of OCD. To conclude, the transdiagnostic nature of UMIs across OCD, BDD, IA/H and EDs could help to explain the phenomenological similarities and comorbidities found among these disorders, supporting the inclusion of IA/H and BDD in OC-spectrum disorders. The QUIT can be reliably used as a screening instrument to detect vulnerable individuals in different contexts and cultures.