This chapter explores the driving forces behind viral evolution, describing analytical methods used to study how viruses adapt and evolve. The chapter reviews tools such as phylogenetic analysis, nucleotide compositional analysis, and codon usage pattern analysis, which help researchers understand viral evolutionary dynamics. Key to this discussion is how codon usage influences viral evolution through selection pressures, mutation rates, and host interactions. Two case studies are presented: one on the evolutionary dynamics of human papillomavirus (HPV) oncogenes and another on codon usage patterns in polyomaviruses across various host species. These case studies highlight how codon bias and nucleotide composition can offer insights into evolutionary pressures and viral adaptation. By applying these tools, researchers can track viral evolution and gain valuable information about viral behavior, including emerging threats. The chapter concludes by emphasizing the importance of codon usage analysis as an essential tool in viral evolution research and its potential for improving public health strategies.