This guide was created in partnership with the National Institute of Justice in order to assist with the selection of chemical and biological defense equipment for emergency workers. This volume of the guide, Volume I, offers technical descriptions that consider the principles of protection. This is the first volume of the series and serves as an information source for all personal protective equipment (PPE). The guide series progresses through Volumes IIa, IIb, and IIc, which each focus on a particular area of PPE, whether respiratory PPE or percutaneous PPE. The introduction explains that the long range plans for defense include the evaluation of existing PPE and the development of a series of documents, including user guides, national standards, and technical reports related to PPE. Since the evaluation process will take several years, this guide was developed in the interim to assist emergency workers in the selection of commercial PPE. The information contained herein was compiled through literature searches and market surveys. The purpose of the guide is to provide information about PPE items to emergency workers, not to make specific recommendations about items. This volume contains background information about the function, components, and levels of PPE; a discussion about chemical and biological agents; an overview of respiratory and percutaneous protective equipment; and an outline of the evaluation techniques used for this guide.