The need for improved methods of measuring abundance of wild animals leads to some suggestions for research on line transects. In particular, extensive field data are needed on the frequency of sightings classified by distance from the observer''s path. A classification of processes underlying transect observations shows the need for models tailored to fit each kind of application, owing to the importance of distinguishing between those effects which depend on the observer''s behavior and those which primarily depend on an animal''s reaction to the observer''s presence. Some approaches for further analysis and field study are outlined.