Mahasi Sayadaw was a Burmese monk in the Theravada Buddhist tradition.

He was a very accomplished practitioner, scholar and writer, and his works in which he describes the details of the stages of insight are the first of this kind in the Buddhist literature, which, coupled with his pragmatic instruction and noting technique, caused a revolution in modern insight practice, creating a great number of highly accomplished practitioners.

Those wishing to read a detailed biography may find one here: http://www.buddhanet.net/mahabio.htm

He trained a number of the people who trained some of the founders of this website, and many practitioners here have been strongly influenced by his work and teachings.