The Internet has grown exponentially during the past 15 years, providing greater access to a wider and more diverse population than before. Because of this development, how people use the Internet to learn about and engage in mental health interventions has also changed, and the Internet now has the potential to be cost-effective, convenient, and reach a broader population than traditional face-to-face interventions. This chapter reviews and summarizes the past fifteen years of progress in online mental health, beginning with the major milestones in Internet history. As the Web took form and grew in popularity, mental health websites allowed people to share basic mental health information and their own personal stories with one another. The use of the Internet as a means for people to learn and share with one another has led to blogging, online self-help support groups, and online mental health interventions, such as personalized, interactive, cognitive-behavioral self-help programs and professional-led online therapy. Emerging uses of the Internet – including its use at disaster sites to help victims of trauma – are discussed, as well as the growth of Web 2.0 and Health 2.0 components.