The article reviews over 40 original studies to establish criteria for comparing viral mutation rates, which are crucial for understanding viral evolution and combating them. The mutation rates of 23 viruses are presented as substitutions per nucleotide per cell infection (s/n/c) and corrected for selection bias using a new statistical method. The rates range from 10^-8 to 10^-6 s/n/c for DNA viruses and from 10^-6 to 10^-4 s/n/c for RNA viruses. A negative correlation between mutation rate and genome size is observed for RNA viruses but requires further experimental testing. Retroviruses do not have lower mutation rates than other RNA viruses. Nucleotide substitutions are four times more common than insertions/deletions (indels). The article also provides estimates of mutation rates per strand copying, which are generally lower than those per cell infection due to multiple rounds of copying per cell. The authors suggest that future studies should minimize the number of cell infection cycles, use large mutational targets, and account for selection bias to improve the accuracy of mutation rate estimates.The article reviews over 40 original studies to establish criteria for comparing viral mutation rates, which are crucial for understanding viral evolution and combating them. The mutation rates of 23 viruses are presented as substitutions per nucleotide per cell infection (s/n/c) and corrected for selection bias using a new statistical method. The rates range from 10^-8 to 10^-6 s/n/c for DNA viruses and from 10^-6 to 10^-4 s/n/c for RNA viruses. A negative correlation between mutation rate and genome size is observed for RNA viruses but requires further experimental testing. Retroviruses do not have lower mutation rates than other RNA viruses. Nucleotide substitutions are four times more common than insertions/deletions (indels). The article also provides estimates of mutation rates per strand copying, which are generally lower than those per cell infection due to multiple rounds of copying per cell. The authors suggest that future studies should minimize the number of cell infection cycles, use large mutational targets, and account for selection bias to improve the accuracy of mutation rate estimates.