A process for liquefaction of natural gas comprising the steps of: (a) providing a refrigeration cycle comprising the steps of: (i) compressing a gaseous refrigerant (550, 540) that consists essentially of nitrogen gas; (ii) cooling the compressed refrigerant from step (i) in heat exchange with a cooling fluid to provide a cooled compressed gaseous refrigerant (510, 525); (iii) work-expanding at least a first portion of the cooled compressed gaseous refrigerant (510, 525) from step (ii) in a first turbo-expander to provide an expanded cooled refrigerant; (iv) reheating the expanded cooled refrigerant from step (iii) in a liquefaction heat exchange system to provide a reheated refrigerant (550, 540); and (v) returning the reheated refrigerant (550, 540) from step (iv) to step (i); (b) providing a natural gas feed stream; (c) compressing the natural gas feed stream; (d) passing the compressed natural gas feed stream directly or indirectly to heat exchange with the expanded cooled refrigerant from step (iii) in the liquefaction heat exchange system; and (e) withdrawing a cooled and at least partly condensed natural gas product from the liquefaction heat exchange system; wherein the term "turbo-expander" refers to a radial or axial flow turbine through which a pressurised gas is expanded; characterised in that the first turbo-expander is used to drive a compressor to compress the natural gas feed stream in step (c). (a) providing a refrigeration cycle comprising the steps of: (i) compressing a gaseous refrigerant (550, 540) that consists essentially of nitrogen gas; (ii) cooling the compressed refrigerant from step (i) in heat exchange with a cooling fluid to provide a cooled compressed gaseous refrigerant (510, 525); (iii) work-expanding at least a first portion of the cooled compressed gaseous refrigerant (510, 525) from step (ii) in a first turbo-expander to provide an expanded cooled refrigerant; (iv) reheating the expanded cooled refrigerant from step (iii) in a liquefaction heat exchange system to provide a reheated refrigerant (550, 540); and (v) returning the reheated refrigerant (550, 540) from step (iv) to step (i); (i) compressing a gaseous refrigerant (550, 540) that consists essentially of nitrogen gas; (ii) cooling the compressed refrigerant from step (i) in heat exchange with a cooling fluid to provide a cooled compressed gaseous refrigerant (510, 525); (iii) work-expanding at least a first portion of the cooled compressed gaseous refrigerant (510, 525) from step (ii) in a first turbo-expander to provide an expanded cooled refrigerant; (iv) reheating the expanded cooled refrigerant from step (iii) in a liquefaction heat exchange system to provide a reheated refrigerant (550, 540); and (v) returning the reheated refrigerant (550, 540) from step (iv) to step (i); (b) providing a natural gas feed stream; (c) compressing the natural gas feed stream; (d) passing the compressed natural gas feed stream directly or indirectly to heat exchange with the expanded cooled refrigerant from step (iii) in the liquefaction heat exchange system; and (e) withdrawing a cooled and at least partly condensed natural gas product from the liquefaction heat exchange system;