The majority of human immunodeficiency virus (HIV) infections in the world are sexually transmitted and quantities of HIV in genital fluids are an important transmission risk-determining factor. Estimating men's sexual HIV infectiousness from blood viral load hinges on the association between HIV in blood plasma (BPVL) and semen viral load (SVL). This article reviews research on the association between BPVL and SVL as reported in 19 empirical studies (N = 1226). Findings yielded a mean correlation between BPVL and SVL of 0.45 (SD = 0.20, median = 0.45, range = 0.07-.64). SVL was generally lower than BPVL, but this pattern was variable across studies. Co-occurring sexually transmitted infections (urethritis), nonsuppressive HIV treatments, and drug resistance account for the variability in observed correlations. HIV disease progression does not reliably influence the association between BPVL and SVL. Research is needed to determine the degree to which BPVL as well as SVL predict HIV transmission.