The Action Notation Modeling Language (ANML) is a relatively new language for specifying planning domain models. These models are created by someone who understands the planning domain and can express the essential aspects of the domain as ANML variables, functions, and actions in a way that supports the generation and execution of safe, efficient, and robust plans. An integrated development environment is essential for enabling programmers and non-programmers to create high-quality, large-scale ANML models more easily. Without such a tool, ANML modeling will remain a tedious and difficult task that can be carried out only by the small number of people who have the necessary specialized programming skills and patience. This, in turn, will severely limit the exploitation of automated planning systems that rely on these models. We propose to develop PM/IDE, an integrated development environment (IDE) for constructing, reviewing, understanding, and testing planning domain models expressed in the Action Notation Modeling Language (ANML) more quickly and effectively. During Phase I, we will characterize the planning domain modeling task to identify the types of analyses and decisions that modelers carry out and the kinds of information they review and assess. Based on this understanding of the task, we will then identify and design useful PM/IDE capabilities and user-system interactions that help people develop ANML models. The system design will specify user-system interactions, software functions and operations, and databases, knowledge bases, and algorithms that implement these functions. We will then develop a limited, proof-of-concept software prototype that demonstrates key parts of the PM/IDE's model editing, query, analysis, and visualization capabilities to demonstrate their utility and feasible implementation.