This　review　discusses　the　recent　experimental　and　computational　investigation　on　the　mechanical　behavior　of　carbon　nanotubes　(CNTs),　CNT　assemblies,　and　their　composites.　The　focus　is　on　the　hierarchical　structure,　deformation　mechanisms,　and　mechanical　properties　of　pristine　CNT/CNT　assemblies　as　well　as　interfacial　properties　and　toughening　mechanisms　of　their　composites.　We　first　introduce　current　research　on　mechanical　performance　of　CNTs　and　CNT　composites.　Then,　we　focus　on　fabrication　technologies,　multiscale　experimental　investigations,　and　numerical　simulations　of　CNT　assemblies,　including　covalent　assemblies　and　non　covalent　assemblies.　Finally,　we　discuss　the　interfacial　properties　of　CNT　fiber　composites.　The　bulk　of　this　chapter　is　devoted　to　current　research　on　the　mechanical　behavior　of　CNT　and　CNT　fibers　as　well　as　the　toughening　mechanisms　of　their　composites.　©　2015　Elsevier　Ltd　All　rights　reserved.