A proposal for the worldsheet string theory that is dual to free N=4 SYM in 4d is made. It is described by a free field sigma model on the twistor space of AdS5 x S5, and it exhibits a psu(2,2|4)_1 affine symmetry. The theory is a natural generalisation of the corresponding model for tensionless string theory on AdS3 x S3 whose description involves a free field realisation of psu(1,1|2)_1. I will explain how our proposal fits into the general framework of AdS/CFT, and review the various checks that have been performed.