Background: The ESP block is a novel ultrasound-guided interfascial technique serendipitously discovered while treating thoracic neuropathic pain in an adult patient. This interfascial block is hypothesised to target the ventral - and dorsal rami of spinal nerves, as the block is performed in a tissue plane deep to the erector spinae muscle. Local anaesthetic is deposited deep to the erector spinae muscle, yet superficial and lateral to the tips of the transverse processes. Moreover, the ESP is also adjacent to the plane between quadratus lumborum and the erector spinae muscles at low thoracic and lumbar levels, and thus it may have a similar mechanism of action with posterior quadratus lumborum block. The therapeutic effect of the ESP block is attributed to the craniocaudal spread of anaesthetic over multiple vertebral levels within the tissue plane. The ESP block carries a lower risk of clinical complications as the needle is inserted into a tissue plane that is a safe distance from that of the parietal pleura, neuroaxis and major neurovascular structures.