Twinning is one of the most popular planar defects in nanocrystals, and it is frequently observed in face-centered cubic (fcc) structured metallic nanocrystals. In this study, copper, silver, and gold nanowires with diameters of 30-50 nm were prepared by electrochemical deposition in "track-etched" polycarbonate membranes. High-resolution transmission electron microscopy (HRTEM) and electron diffraction were used to study the microstructure of the nanowires. Microtwins were observed to be dependent on growth orientation in the single-crystal metal (copper, silver, and gold) nanowires. In single-crystal nanowires with a [111] growth orientation, both primary (111)[11 (2) over bar] and secondary ((1) over bar(1) over bar1)[112] twins were observed, whereas in the wires with a [112] growth orientation, lengthwise twins and stacking faults were the dominant features. No twinning was observed in nanowires that grew along the [100] and [110] directions. The crystallography of the twins was characterized by HRTEM and electron microdiffraction. The twinning mechanism (nucleation and growth) is discussed in relation to a two-dimensional (2D) nucleation and growth mechanism for the single-crystal nanowires.