This document provides guidance to industry and FDA Staff on the underlying principles of human factors (HF) studies during the development of combination products as defined under 21 CFR Part 3.  This guidance describes Agency recommendations regarding HF information in a combination product investigational or marketing application and clarifies the different types of HF studies; the recommended timing and sequencing of HF studies; and how HF studies are part of the process to maximize the likelihood that the combination product user interface is safe and effective for use by the intended users, uses, and environments.  In addition, the guidance describes how HF studies relate to other clinical studies.  The guidance also provides process considerations for HF information in investigational or marketing applications to promote development and timely review of safe and effective combination products.