Ever since the introduction of the polymerase chain reaction (peR) in 1986, morphologists, whose interests lie in the analysis of intact tissue structures, have been attempting to adapt this technique to intact cells or tissue sections to detect low copy numbers of DNA or RNA in situ while preserving tissue morphology. The significance of this objective is obvious. A technique finally materialized in 1990 when Dr. Ashley T. Haase and coworkers published results that used multiple prim­ ers with complementary tails in intact cells. Since then, a number of laboratories have successfully developed their own versions of the technique. In situ peR is now a well-recognized method that permits the detection of minute quantities of DNA or RNA in intact cells or tissue sections. As a result, morphological analysis of those target nucleotide sequences becomes possible. As anticipated, this ad­ vancement has led to significant improvement in our understanding of many nor­ mal and abnormal conditions, and its impact is becoming more evident as time passes. In situ peR has the characteristics of a new landmark in morphologic technol­ ogy-it is scientifically fascinating and technically challenging. In essence, it is a combination of in situ hybridization and conventional peR. The wealth of literature, experience and prools for the two latter techniques can be applied to in situ peR. In situ peR also has its own unique aspects that were not addressed by the other two techniques.