Two-dimensional (2D) boron nitride (BN) nanosheets, the rising material stars, have unique properties and amazing functionalities. The concise history of diverse synthesis routes of BN nanosheets is briefly summarized here; and the recent development towards mass production of BN nanosheets, i.e. "chemical blowing" technique relying on blowing molten polymer precursors into large polymeric bubbles and subsequent annealing into BN bubbles/nanosheets, is focused on. The abundant BN nanosheets enable their applications, especially in the representative polymeric composites with BN nanosheet additives, as particularly highlighted in this review. The highly thermoconductive insulating BN-filled composites are thus envisaged as high-performance packaging materials for electrical circuits.