3.7V Lithium polymer battery connector and built in battery charging ATSAMD21G18 ARM Cortex M0 processor - the same one used in the Arduino Zero 256K of FLASH (8x more than the Atmega328 or 32u4) and 32K of RAM (16x as much) Built in USB so it has USB-to-Serial program & debug capability built in with no need for an FTDI-like chip