Bisulfite sequencing is a sequencing technique used to identify methylation on cytosine nucleotides.
The DNA sample is treated with sodium bisulfite, causing unmethylated cytosines to be converted to uracil. This allows for sequencing with next generation technologies, such as Illumina, to identify methylated of the DNA and subsequently transcriptionally active regions (by comparison with the original sequence from before bisulfite conversion).
With the advent of third generation sequencing technologies, such as PacBio and Oxford Nanopore, it is possible to identify methylation during the normal sequencing process, negating the requirement for bisulfite conversion (or any other library preparation step). This allows for the capture of more data about the DNA of interest, providing greater information to study.