The internal anal sphincter is the inner of the two anal sphincter muscles, a caudad continuation and expansion of the inner circular layer of smooth muscle in the rectal wall. It is under autonomic control and is tonically contracted, providing a closed anus in between defecations. When the rectum fills with stool and intra-rectal pressure rises, there is a reflex relaxation of the internal sphincter, allowing rectal contents to contact the sensitive anal transitional epithelium (the recto-anal inhibitory, or “sampling” reflex). The high intrarectal pressure is accompanied by a sensation of rectal filling, and the sampling reflex contributes a sense of the nature of the rectal contents. The external anal sphincter can then be used to close the anus so that control is maintained until defecation. The importance of this mechanism is seen in paraplegic patients where the sensation of rectal filling is not appreciated and the external sphincter is not contracted: defecation is automatic. This chapter is about internal anal sphincter achalasia, and uncommon condition where the internal anal sphincter is in a permanent state of spasm.