Highly active antiretroviral therapy has successfully controlled HIV replication in many patients. The treatment effectiveness depends on many factors, such as the intracellular stages of the viral replication cycle inhibited by antiretroviral drugs and the pharmacodynamics of drugs. In this talk, I will present some recent work on studying HIV dynamics under treatment. Using multi-stage models, we will show that different drug classes have different influence on HIV decay dynamics. Using multi-scale models, we will show that pharmacodynamic profiles of drugs can significantly affect the outcome of either early or late HIV treatment.