This review contrasts the chemistry of the allyl and crotyl Grignard reagents. These two organometallics are extremely reactive and in many instances are insensitive to steric hindrance in the substrate. Accordingly, they provide a useful synthetic tool for introducing a three- or four-carbon atom chain containing a site of unsaturation. The review includes: 1. Methods of preparing the two Grignard reagents, 2. a discussion of their structure, 3. transformations these reagents undergo, including displacement reactions, additions to carbonyl, azomethine, and olefinic linkages.