The invention relates to methods for indexing samples during the sequencing of polynucleotide templates, resulting in the attachment of tags specific to the source of each nucleic acid sample such that after a sequencing run, both the source and sequence of each polynucleotide can be determined. Thus, the present invention pertains to analysis of complex genomes (e.g., human genomes), as well as multiplexing less complex genomes, such as those of bacteria, viruses, mitochondria, and the like.