It is expected that if you agree to review a particular proposal or paper you have the appropriate expertise. While preliminary judgments are made by the individual who assigns the review to you, the final responsibility as a reviewer is yours. Interdisciplinary or multidisciplinary projects can be somewhat of a challenge in this regard. In some cases, you may be instructed to review only those portions of the paper or application that are within your area of expertise. It is useful to the editor or program officer if you spell out explicitly in your review what portions are outside your area of expertise.