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SUMMARY Instream flow determination and implementation involves a wide range of agencies, professions, and interests. Decisions made regarding streamflow allocations require the evaluation of numerous factors over a range of events. Certain factors are used to judge the benefits and liabilities resulting from a particular management practice. These factors are called decision variables, and may range from tons of corn to kilowatt hours of electricity to square feet of fish habitat. The role of technical information in the decision process is to quantify changes in the decision variable in response to various management alternatives. The Instream Flow Incremental Methodology (IFIM) is designed for iterative problem solving in this context of decisionmaking. The decision variable generated by the IFIM is total habitat area for fish or food organisms. Habitat, as computed by the IFIM, incorporates longi- tudinal changes in channel characteristics, streamflow, water quality, and temperature. These factors are called macrohabitat features, and determine the longitudinal distribution of various species. Habitat also includes the distribution of hydraulic and structural features comprising the actual living space of the organism, called microhabitat. The total habitat available to a species at any streamflow is the area of overlap between available microhabitat and suitable macrohabitat characteristics. All applications of the IFIM begin with a five-step scoping process. The first step is to define the problem to be addressed and to rigorously define the objectives of the study. The objectives must anticipate the kinds of information the study is to provide, and must sometimes be negotiated to fit the availability of time, money, and personnel. The second and third steps in the scoping process are designed to place bounds on the problem. The geo- graphical extent of the study area is determined, including the length of mainstem river to be considered and whether or not tributaries are to be included in the analysis. Project impact studies differ from instream flow studies, in this respect, because tributaries are not evaluated in the former unless they are directly affected by the project. The third step in the scoping process is a determination of the environmental variables that must be analyzed and those that can be safely ignored. This process actually involves two determinations: an evaluation of present macrohabitat conditions and an estimation of these conditions with the project in place. Several screening techniques are presented to help the investigator judge the necessity for quantitative analysis of a specific environmental variable. The fourth step in the scoping process is the selection of appropriate evaluation species. These may include game, sport, or commercial species, endangered species, indicator species, food organisms, and major competitors of the management objective species. This step is important because all interpretations regard- ing the significance of an environmental change are based on consequences to the evaluation species. The final scoping activity is to describe temporal variations in habitat usage by each evaluation species. This step determines the life stages and types of mi.crohabitat that must be evaluated during each month. Habitat characteristics are measured at study sites within the geograph- ical study area. Some study sites are established to measure or monitor v