Poka-yoke is the Japanese word for mistake-proofing. Mistake-proofing is a very powerful set of technique that either keep defects from occurring or indicating immediately when a defect does occur. It relies on creativity and common sense to create low cost, effective inspection devices. Like many quality tools, mistake-proofing is not a solution for every kind of defect or quality problem. Moreover, some mistake-proofing techniques are more effective than others for certain circumstances. The basics of mistake-proofing are reviewed. Factors when mistake-proofing works well, and when it does not are discussed in general and for each inspection technique. Several examples are provided.