An Internet-based self-screening instrument for assessing psychological stress reactions was created in the aftermath of the 2004 tsunami affecting the countries around the Indian Ocean. Named the Online Self Evaluation Tool (ONSET), it permitted any affected subjects to self-evaluate whether further clinical assessment by trained mental health professionals was needed. This chapter outlines all experiences and existing scientific evidence guiding the development of ONSET, reports the results of the use of this instrument for the tsunami victims, and discusses why early screening after disasters is important. From the entire sample of ONSET users, 45% of respondents reported possible posttraumatic stress disorder symptoms, 24% reported possible depression, 19% potentially had anxiety disorders, and 27% reported an increased use of psychotropic substances. Additionally, a total of 18% of ONSET users reported experiencing suicide ideations. Findings support the recommendation that future research on mental health issues should put more emphasis on the importance of early monitoring and assessment to detect early PTSD and other reactions in the wake of natural disasters.