Provided herein are methods for the assembly of a polynucleic acid sequence that is at least partially carried out on a microfluidic device; methods for the preparation of a library of polynucleic acid sequences; microfluidic devices; methods for designing nucleic acid sequences; methods for planning the assembly of a polynucleic acid sequence from a plurality of nucleic acid sequences; systems comprising components for carrying out these methods; computer programs which, when run on a computer, implements these methods; and computer readable medium or carrier signals encoding such a computer program.