For　discrete-time　unknown　nonlinear　dynamic　systems,　an　online　optimal　tracking　control　scheme　is　developed　in　this　paper,　which　is　implemented　by　direct　heuristic　dynamic　programming　(HDP).　To　start　with,　a　solution　procedure　is　presented　for　tracking　problems　of　discrete-time　nonlinear　systems.　Then,　the　basic　structure　and　mechanism　of　the　direct　HDP　algorithm　are　described　in　detail,　including　the　critic　network　and　the　action　network.　Moreover,　based　on　the　direct　HDP　algorithm,　the　implementation　process　of　neural　networks　is　provided.　Finally,　the　direct　HDP　algorithm　is　applied　to　enable　the　original　system　to　track　the　reference　trajectory.　The　effectiveness　of　the　algorithm　is　proved　in　solving　tracking　problems　and　online　optimal　tracking　control　is　achieved.