Infection by human immunodeficiency virus type I (HIV-1) requires virus particle binding to host cell-surface receptor CD4 via the viral envelope glycoprotein gp120. HIV-1 therapy and prevention efforts involve development of mimetic or recombinant gp120 vaccines or deployment of antiviral agents that target specific epitopes of gp120. The unliganded conformational state of gp120 is closed, whereas the CD4-bound state is open. However, in between, there exist dynamic conformational states, indicating intrinsically flexible region(s) of structural dynamics, imposing a structural challenge for developing drug or antibody targets. Known conformational states of gp120 were determined by X-ray crystallographic and cryo-electron microscopy, and neither method captures the population of gp120 species arising from conformational plasticity, motions, and transitions. gp120 plasticity brings up several important questions. How will differences in conformation affect receptor binding, antibody recognition, and neutralization? Which regions are crucial for gp120 structural plasticity? How could structural dynamics influence HIV-1 evasiveness against host immunity and drugs or vaccines, and facilitate the viral entry into its host? This review explores the structural constraints presented by conformational states of the glycoprotein to antibodies or drugs and how these conformational states provide structural avenues for the virus to escape neutralizing agents and evade host immunity.