This chapter explains the importance of long-term data to ecological studies and the needs of creating a way to provide accessible and comparable multi-site long-term data. The Earth’s environment is changing in many ways at local, regional, and global scales. Dramatic changes have occurred in climate, land cover, and habitat availability over the past several centuries. Long-term data (> 10 years) are needed to assess the rate and direction of change, to distinguish directional trends in these changes (e.g., persistent increases or decreases) from short-term variability (e.g., multi-year cycles), and to forecast environmental conditions in the future.  It also sets up the stage for the rest of the chapters in the book, presenting the purposes, audience and practical applications of the book, explaining how the study sites, variables, and data sets are selected, and how the data are presented and analyzed, and laying out the organization of the book.