In situ transmission electron microscopy (TEM) nanomechanicaltesting has benefited from a number of recent technical developments related toboth how deformation is imaged and how deformation is induced and measuredinside a TEM instrument. These developments have led to new insights into thedeformation mechanisms of a wide range of metals and alloys, as well asmeasurements of the unusual mechanical properties of small-scale objects such aswhiskers and nanocrystals. Herein, we describe this recent progress throughselected highlights of recent findings on the dynamic behavior of defects suchas dislocations, twins, and grain boundaries.