The invention provides a polymyxin compound of formula (I) and salts, solvates and protected forms thereof, pharmaceutical compositions comprising the compounds of formula (I), and the use of the compounds and compositions in methods of treatment, such as methods for the treatment of microbial infections. The compounds of formula (I) are represented thus: wherein —R15 is a group: and —R16 is hydrogen; —R17 is hydrogen -L- is a covalent bond or methylene; and —Ar is optionally substituted aryl. The groups —X—, —R1, —R2, —R3, —R4, and —R8 are as defined herein.