The National Institute of Standards and Technology (NIST) has undertaken a project for the U.S. Environmental Protection Agency to develop and demonstrate evaluation methods to relate shelter airtightness to the performance of shelter-in-place approaches for airborne chemical, biological, and radiological (CBR) protection of building occupants. The focus of this effort is on short-term sheltering, on the order of hours, rather than longer-term sheltering, which generally employs filtration and air cleaning equipment to supply clean air to the occupants of the space. This project has consisted of the following tasks: a literature review of SIP strategies and performance issues; development of a study plan for testing SIP airtightness evaluation methods; implementation of the study plan through a combination of experiments and simulations; and, finally, development of recommendations on SIP evaluation and possible performance criteria for candidate SIP spaces.