We are conducting a research study because we are trying to learn more about what occurs before and during post-exertional malaise for individuals with Myalgic encephalomyelitis (ME) or chronic fatigue syndrome (CFS), as well as how these individuals define the term post-exertional malaise. We are asking you to be in the research because you have a diagnosis of ME or CFS, speak English, and are 18 years of age or older. If you agree to be in this study, you will be asked to complete a survey. The survey will include questions about your post-exertional malaise experiences related to your ME or CFS. We will also collect some personal and demographic information about you, such as age, gender, race, marital status, income, level of schooling, disability information, and work status. The full survey can be completed online. If there is a question you do not want to answer, you may skip it.