This chapter describes the method of Active Information Search (AIS), whose goal is to investigate spontaneous information acquisition while avoiding the limitations that come with pre-structuring the decision task. The method of AIS was developed to understand information needs in decision making with as little demand effects as possible. The proper pre-testing of an AIS scenario is a prerequisite to achieve adequate reliability and validity. The basic as well as the list version of AIS can be translated into a computer program. The experiment can be administered in the laboratory but also on the internet. For instance, basic AIS may be applied with only little programming effort via the internet with chat or messaging programs. The AIS method helps to understand reasoning, judgment and decision making, or complex problem solving, by providing process measures of the type, quantity, and sequence of information people are interested in.