HIV replication occurs principally in activated CD4+ T cells and macrophages. The HIV-1 Tat protein is essential for HIV replication and requires a cellular protein kinase activity termed TAK / P-TEFb, composed of CDK9 and cyclin T1, for its transactivation function. This article reviews recent work indicating that under some circumstances TAK / P-TEFb is likely to be limiting for HIV replication in CD4+ T cells and macrophages, and discusses mechanisms of regulation of the TAK / P-TEFb subunits in these cell types. In resting CD4+ T lymphocytes, TAK / P-TEFb function is low. Following lymphocyte activation, even under conditions of minimal activation in which activation markers and cellular proliferation are not induced, both CDK9 and cyclin T1 mRNA and protein levels are increased, leading to an induction of TAK / P-TEFb kinase activity that correlates with increased viral replication. In macrophages, regulation of TAK / P-TEFb involves mechanisms distinct from those in lymphocytes. In freshly isolated monocytes, CDK9 protein levels are high, while cyclin T1 protein levels are low to undetectable. Cyclin T1 protein expression is up-regulated during early macrophage differentiation by a mechanism that involves post-transcriptional regulation. Later during differentiation, cyclin T1 expression becomes shut off by a post-transcriptional mechanism, and this correlates with a decrease in Tat transactivation. Interestingly, cyclin T1 can be re-induced with lipopolysaccharide (LPS). These findings suggest that changes in cyclin T1 expression can influence HIV-1 replication levels in monocytes and macrophages. Important areas for future research on Tat and TAK / P-TEFb function are discussed.