Methods and devices are provided for activating brown adipose tissue (BAT) using electrical energy. In general, the methods and devices can facilitate activation of BAT to increase thermogenesis. The BAT can be activated by applying an electrical signal thereto that can be configured to target sympathetic nerves that can directly innervate the BAT. The electrical signal can be configured to target the sympathetic nerves using fiber diameter selectivity. In other words, the electrical signal can be configured to activate nerve fibers having a first diameter without activating nerve fibers having diameters different than the first diameter. Sympathetic nerves include postganglionic unmyelinated, small diameter fibers, while parasympathetic nerves that can directly innervate BAT include preganglionic myelinated, larger diameter fibers. The electrical signal can be configured to target and activate the postganglionic unmyelinated, small diameter fibers without activating the preganglionic myelinated, larger diameter fibers.