The invention relates to encoding and decoding variable length instructions. Methods of encoding and decoding are described which use a variable number of instruction words to encode instructions froman instruction set, such that different instructions within the instruction set may be encoded using different numbers of instruction words. To encode an instruction, the bits within the instructionare re-ordered and formed into instruction words based upon their variance as determined using empirical or simulation data. The bits in the instruction words are compared to corresponding predicted values and some or all of the instruction words that match the predicted values are omitted from the encoded instruction.