The present invention provides, among other things, methods of purifying messenger RNA (mRNA) including the steps of subjecting an impure preparation comprising in vitro synthesized mRNA to a denaturing condition, and purifying the mRNA from the impure preparation from step (a) by tangential flow filtration, wherein the mRNA purified from step (b) is substantially free of prematurely aborted RNA sequences and/or enzyme reagents used in in vitro synthesis.