What are the basic principles of PCR primer design?

Effective PCR primers typically follow these guidelines: length of 18-25 nucleotides, GC content of 40-60%, balanced distribution of bases, melting temperature (Tm) of 55-65C with both primers within 5C of each other, ending with G or C at the 3' end (GC clamp) for stable binding, avoiding self-complementarity and primer dimers, and avoiding runs of single nucleotides. Primers should be specific to the target with no significant off-target binding sites. Online tools (Primer3, NCBI Primer-BLAST) help design and validate primers against genome databases.