Contextual inquiry is a user-centered design process that involves observing and interviewing users while they perform tasks in their natural environment. It is a type of ethnographic field study that provides detailed insights about work practices, behaviors, and the interactions with complex systems. The method is often used during the early stages of product development and is especially well-suited for understanding users' interactions with complex systems.

The primary goal of a contextual inquiry is to capture detailed information about how users interact with a product in their normal work environment. This includes observing user behavior and discussing those activities with the user while they're at work.

Contextual inquiry is based on four guiding principles:

1. Context: The research is conducted in the users’ natural environment.
2. Partnership: The user and researcher work together to understand the tasks and issues.
3. Interpretation: The researcher develops a shared understanding of all important aspects of the work with the user's feedback.
4. Focus: The researcher understands the purpose of the research project and seeks relevant information.

The process of conducting a contextual inquiry usually involves three phases: the introduction, the body of the interview, and the wrap-up. In the introduction, the researcher introduces themselves and gains an overview of the user's work. The body of the interview involves observing the user's work and discussing their activities. Finally, during the wrap-up, the researcher summarizes the findings, offering the user a chance to provide final corrections and clarifications.

Contextual inquiry is a valuable tool in various domains, including the design of complex systems, understanding users' interactions, and gaining insights into expert users' point of view. However, it might not be particularly useful for targeted design tasks such as redesigning an ecommerce product page or testing a newsletter signup form on a website. These interfaces don't typically require in-depth thought processes or an underlying body of knowledge that UX professionals must understand to design them.

After completing contextual inquiry sessions, researchers and designers should come together to share findings and interpret the results of the interviews. Methods such as affinity mapping can help the team align on patterns and themes. The result is a shared understanding of users’ work processes, mental models, and common behaviors, which can then be used to design solutions for their customers.