Assessment of eutrophication of lakes in watersheds undergoing development is facilitated by estimates of (1) background phosphorus (P) loading and concentration, (2) present-day P concentrations and amounts and sources of P loadings in excess of background levels, (3) the sensitivity of the lakes to future increases in P loading, and (4) relationships between P concentration and other factors that determine lake water quality. Methods have been developed for making such estimates for lakes in the Puget Sound Region based on data already available for most lakes in the region. Background P loadings were computed from P concentration data for 24 undeveloped lakes in the region using a mass balance model, and predictive relationships were developed from these loadings to estimate background loading for other lakes. The standard error of estimate for background loadings and concentrations averages 25 percent for most lakes in the region. Present-day P loadings were then estimated from measured P concentrations for 28 lakes in basins containing residential land. Differences between present-day and background loading were attributed to land use changes. P loadings from septic systems, computed as the difference between total present-day loading and the sum of background and residential runoff loading, were found correlated with the presence of old homes around the lakes (r2= 0.36). The regression relationship expressing this relation can be used to estimate septic-system loading for other lakes. If necessary, P loading from agricultural land can then be estimated on a lake-by-lake basis as the difference between total present-day loading and the sum of background and both types of residential area loading. Methods are presented for estimating the reliability of all estimates, which varies. These empirical relations allow approximations of the cumulative impact that development has had on P loading and the amounts of loading from generalized land use categories. The mass balance P model also estimates lake sensitivity to future increases in P loading. Finally, predictive relationships were developed between P concentrations and Secchi disk transparency and chlorophyllaconcentrations, two key indicators of lake water quality related to eutrophication, based mainly on data for 17 well-studied lakes in the region.