Current environmental concerns foster a strong interest in extracting polymers and building-blocks from lignocellulosic biomass. Among these, cellulose nanocrystals (CNCs) have raised both academic and industrial interest because of their advantageous properties. Alongside their high mechanical properties, low density, high surface area and biodegradability, CNCs also lend themselves to functionalization and self-assembly into interesting structures. As a result, they have shown great potential in a wide range of applications from the automotive industry, to packaging materials and tissue engineering. CNCs are mainly produced using a top-down hydrolytic approach from biomass. Production methods vary significantly, aiming at maximum CNC yield and quality, while also considering environmental impact and economics. This manuscript reviews the traditional and more recent routes for producing cellulose nanocrystals. An approach to compare effectiveness and environmental impact of CNC production from Brønsted acids is also proposed and implemented to compare the various production routes.