W e use monitoring data routinely in our daily lives; we monitor the stock market, the weather, our blood pressure, and baseball statistics. But, does monitor-ing have a place in environmental science? Common crit-icisms of environmental monitoring are (1) that it is not really science, but merely a fishing expedition that diverts funds from "real" science, (2) that most monitoring data are never used, and (3) that we can't possibly know today what critical questions will need to be answered in the future. Some people feel that monitoring has no place in rigorous environmental science, and mindless monitoring gives the discipline a bad reputation. Who needs it? In this paper, we evaluate these common perceptions of environmental monitoring and discuss the characteristics of successful and unsuccessful monitoring programs. We define environmental monitoring as a time series of mea-surements of physical, chemical, and/or biological vari-ables, designed to answer questions about environmental change. These measurements may be taken at one or multiple locations. The meaning of "long term" depends on the time scale of the ecological process of interest, but in this paper we focus on datasets that span decades. Our discussion is particularly relevant, given the budgetary constraints on current monitoring and the ongoing debate regarding the opportunities, limitations, and costs associated with the establishment of national environ-mental observatories in the US, for example the National Ecological Observatory Network (NAS 2004), the Hydrologic Observatories, and the Cooperative Large-scale Engineering Analysis Network for Environmental Research (NRC 2006).