Planning and monitoring a trip is a common but complicated human activity. Creating an itinerary is nontrivial because it requires coordination with existing schedules and making a variety of interdependent choices. Once planned, there are many possible events that can affect the plan, such as schedule changes or flight cancellations, and checking for these possible events requires time and effort. In this paper, we describe how Heracles and Theseus, two information gathering and monitoring tools that we built, can be used to simplify this process. Heracles is a hierarchical constraint planner that aids in interactive itinerary development by showing how a particular choice (e.g., destination airport) affects other choices (e.g., possible modes of transportation, available airlines, etc.). Heracles builds on an information agent platform, called Theseus, that provides the technology for efficiently executing agents for information gathering and monitoring tasks. In this paper we present the technologies underlying these systems and describe how they are applied to build a state-of-the-art travel system.