When people navigate through streets and cities, they use various kinds of information such as maps, landmarks, and guidance from other people. The purpose of this study is to examine when and why people make use of such external resources. In the experiment, subjects were asked to go to a local town where they had to visit six destinations using information obtained from either a route guidance service or from pedestrians. The routes they walked along were recorded by a Personal Handy phone System with location information (LI-PHS) and a video tape recorder. Their navigation processes and their verbal protocols were recorded and an analysis of when and why they relied on other people (route guidance and pedestrians) was made. The results indicate that people utilize various external resources, including human resources and situational cues which are both stable (e.g. finding a building) and unstable (e.g. finding a school girl when looking for a school), to choose their routes. The use of a route guidance service with mobile phones and location tracking devices (LI-PHS) to study human navigation processes is also discussed.