Over the years since 1981 we have designed and implemented a balanced and integrated data collection program (online data catalog at http://lter.limnology.wisc.edu/catalog.html). We selected lakes and measurements to address important interdisciplinary questions regarding the ecology and management of lakes from a long-term perspective at individual lake, multiple lake, lake-district and regional scales. We focus our data collection on two sets of lakes and their surrounding landscapes. One set is in the forested and tourism-dominated Northern Highland Lake District in northern Wisconsin, the other is in the agricultural- and urban-dominated landscape in and near Madison in southern Wisconsin. Both regions have a substantial history of ecological research dating back to about 1900. In northern Wisconsin, beginning in 1981, we focused on a suite of seven primary lakes and surrounding terrestrial areas linked through a common groundwater and surface water flow system and sharing a common climatic, edaphic, and biogeographic regime. The lake set includes oligotrophic, dystrophic, and mesotrophic lakes chosen to represent marked differences in size, morphometry, habitat diversity, thermal and chemical features, species richness and assemblies, and position in the groundwater flow system. In 1994, we added four primary study lakes in southern Wisconsin. These four eutrophic lakes were chosen in a 2x2 design of urban vs. agricultural setting and headwater vs. lower in the landscape. Substantial historical data are available on these lakes. In addition to the primary lakes we also have a set of secondary lakes for which less complete information is collected. These lakes are used for comparisons with primary lakes on specific research questions. We also collaborate with two Canadian groups with similar data on two other lake districts, the Experimental Lakes Area in western Ontario and the Dorset Research Centre in eastern Ontario. Collectively, the data and research programs at these four lake districts afford a unique opportunity for analyses of the Western Great Lakes region. Our sampling program allows comparisons of parameters and processes among seasons, years, lakes, and lake districts. We sample most major physical, chemical and biological components with sampling frequencies tuned to the dynamics of individual parameters. We sample most intensively at four key times of the year: spring overturn, maximum stratification in summer, fall overturn, and winter stratification. Complete cation-anion balances are determined at these times. Nutrients, pH, inorganic and organic carbon are sampled every two or four weeks, depending on the lake and the nutrient. Temperature, dissolved oxygen, chlorophyll a, light penetration, and zooplankton abundance are sampled every two weeks during the open-water season and every five weeks under ice cover. Primary production rates are measured every two weeks from selected lakes and samples for phytoplankton community composition are collected six times throughout the year. Parameters that vary over longer time scales are measured annually in August. These include macrophyte distribution and abundance, fishes (abundance, biomass, and community structure), and benthic invertebrate abundance. Groundwater levels in selected wells are measured monthly and groundwater chemistry from a subset of these wells is measured annually. We have a series of instrumented buoys on selected primary lakes, including a raft on Sparkling Lake for measurements of evaporation, wind stress, and high-resolution thermal structure, and an instrumented buoy on Trout Lake for vertical profiling. We plan to develop and deploy small buoys measuring pCO2, oxygen, and water temperature for estimates of gross primary production, respiration, ecosystem productivity, and carbon flux to the atmosphere on each primary lake. In addition, we maintain an automated land-based weather station at the local airport 10 km from Trout Lake. We have access to National Weather Service data from the Madison airport. In addition to providing comprehensive limnological data, this sampling program positions us to detect invading exotic species in our primary lakes. Potential new invaders include many European species that have reached the Laurentian Great Lakes (www.seagrant.umn.edu/exotics/index.html). These large lakes now act as a nearby source of colonists including fishes (ruffe, rainbow smelt, rudd, round goby, etc.), zooplankton ( Bythotrephes cederstroemi, Eurytemora affinis, etc.), molluscs (zebra mussels, fingernail clams, and a variety of snails), a macrophyte (Eurasian watermilfoil), and a highly toxic cyanobacterium now invading North America ( Cylindrospermopsis raciborskii). We have designed our sampling so that introductions of these or other invading species will be discovered early and we can implement specific research activities to understand consequences of these introductions. To provide basic information about the terrestrial landscapes surrounding our study lakes, we have developed a geographic information system that includes data layers on land use/land cover, soils, topography, roads, and other landscape features. We have a particularly strong foundation of spatial data on land use and land cover, including a statewide pre-settlement vegetation database; detailed, large-scale historical land use/land cover databases from the 1930’s, 1960’s, and 1990’s for watersheds or riparian zones of selected study lakes; and the statewide WISCLAND land cover database. We maintain an extensive archive of airborne and satellite imagery for both the northern and southern lake regions. The core of this archive consists of Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) images from 1984 to the present. Other image data sources available to researchers at the NTL site include the EO-1/Advanced Land Imager and Hyperion imaging spectrometer, highresolution (1m to 4m) IKONOS images for the Madison and Trout Lake areas, daytime and nighttime thermal images from ASTER, the NASA ATLAS airborne multispectral and thermal scanner, and the Shuttle Imaging Radar-C (SIR-C) system.