The brachial plexus is formed from the anterior primary rami of C5 to T1 and provides all motor and sensory innervation to the upper limb. The nerves that make up the plexus lie in a sheath that extends from the tubercles of the transverse processes of the cervical vertebrae to the axilla; it is within this sheath that we would inject our local anaesthetic. As these five roots pass between scalenus anterior and medius, C5 and C6 join to form the upper trunk, the root of C7 continues as the middle trunk and C8 and T1 unite to form the lower trunk. The interscalene approach aims to anaesthetise the roots of the plexus.