The duty not to interfere in the internal affairs of the receiving State was not included in the original draft by the ILC’s Special Rapporteur, but was introduced through a later amendment. Given the significance which incidents of (alleged) interference had attained even then, this is a somewhat surprising development. In contemporary diplomatic relations, such charges play an important role and affect a wide variety of fields, ranging from criticism of the receiving State, human rights monitoring, support given to factions in that State, etc. This chapter explores the concept of interference, but it also reflects on legitimate interests on the side of the sending State which may allow (and even compel) a diplomatic agent to take measures which his hosts may consider interference. The chapter also suggests mechanisms, including the employment of proportionality, which are capable of mediating between the interests advanced on both sides of the divide.