One of the most important factors affecting the quality of PCR is the choice of primers. In general, the longer the PCR product the more difficult it is to select efficient primers and set appropriate designing primers, and in general, the more DNA sequence information is available, the better the chance of finding an optimal primer pair. Efficient primers can be designed by avoiding the following flaws: primer-dimer formation, self-complementarity, too low Tm of the primers, and/or their incorrect internal stability profile. Tips on subcloning PCR products, calculating duplex stability (predicting dimer formation strength), and designing degenerate primers are given.