The chapter introduces *Primer3*, a computer program designed for designing PCR primers and sequencing primers. It covers the program's capabilities, including its ability to select primers for various applications such as creating sequence tagged sites (STSs) and amplifying sequences for single nucleotide polymorphism (SNP) discovery. *Primer3* can also design oligonucleotide hybridization probes and consider multiple factors such as melting temperature, length, GC content, and secondary structure. The chapter discusses two main ways to use *Primer3*: through its web interface and as a software component. The web interface is suitable for casual users, while the software component, *primer3_core*, is more efficient for high-volume users. The chapter provides detailed instructions on how to install and use *primer3_core* as a software component, including examples of using it in Unix pipes and Perl scripts. Additionally, the chapter highlights the diversity of primer design applications and the flexibility of *Primer3* in handling different constraints and objectives. It also addresses common issues when *Primer3* cannot find acceptable primers, suggesting strategies to relax constraints or adjust the objective function. The chapter concludes with acknowledgments and references, detailing the funding sources and contributions from various individuals and organizations.The chapter introduces *Primer3*, a computer program designed for designing PCR primers and sequencing primers. It covers the program's capabilities, including its ability to select primers for various applications such as creating sequence tagged sites (STSs) and amplifying sequences for single nucleotide polymorphism (SNP) discovery. *Primer3* can also design oligonucleotide hybridization probes and consider multiple factors such as melting temperature, length, GC content, and secondary structure. The chapter discusses two main ways to use *Primer3*: through its web interface and as a software component. The web interface is suitable for casual users, while the software component, *primer3_core*, is more efficient for high-volume users. The chapter provides detailed instructions on how to install and use *primer3_core* as a software component, including examples of using it in Unix pipes and Perl scripts. Additionally, the chapter highlights the diversity of primer design applications and the flexibility of *Primer3* in handling different constraints and objectives. It also addresses common issues when *Primer3* cannot find acceptable primers, suggesting strategies to relax constraints or adjust the objective function. The chapter concludes with acknowledgments and references, detailing the funding sources and contributions from various individuals and organizations.