Visual survey techniques are used widely to estimate abundances of target organisms in terrestrial and aquatic environments. There are a number of methodological 'errors' in almost all applications of visual surveys. Given the dependence of all visual survey data on the skill and technique of the observer, one potentially important source of imprecision andlor bias is vanation among and within observers. In studies involving large amounts of fieldwork over great geographic range and many years, it is inevitable that observers will change from place to place and through time at any single site as they are replaced or gwn experience. We present the results of 3 observer tralningkalibration exercises that lnd~cate that observational studies in which multiple observers must be employed may be subject to considerable observer-related biases and imprecision. We found that careful training and calibration of observers ameliorated such effects for most taxa, but non-trivial levels of bias for some taxa and imprecision m estimates for several taxa remained even after thorough training. It is essential that the influence of observer bias and impreclslon be well documented in multi-observer monitoring studies, so that (spurious) patterns related to differences among observers can be distinguished from real spatial or temporal patterns in the environment.