The object of this thesis is to examine the legality of nonforcible counter-measures in bilateral contexts. The major questions addressed are as follows: (i) do counter-measures constitute an autonomous category of justification for wrongful conduct? (ii) if so, what are the conditions of the legality of that category? (iii) what are the more significant collateral constraints on the legality of countermeasures? and (iv) what is the extent to which policy considerations contribute towards the determination of the legality of counter-measures? The study begins with an Introduction which indicates the scope and the outline of the thesis, the terminology employed, and the approach adopted. Chapters One and Two deal with the historical development of the law of reprisals with special reference to non-forcible counter-measures. Chapter Three is entitled "The status of non-forcible counter-measures in customary international law since 1945: a preliminary sketch". Chapters Four, Five,and Six are devoted to examining the conditions of the legality of non-forcible counter-measures, viz., (i) breach, (ii) prior demand for reparation; and (iii) proportionality. Chapter Seven examines some of the more significant collateral constraints on the legality of counter-measures. In Chapter Eight an attempt will initially be made to examine the circumstances in which the performance of obligations under a treaty may be withheld as a counter-measure. The conclusions reached will then be compared and contrasted with the regime established by the Vienna Convention on the Law of Treaties. Chapter Nine considers the legality of counter-measures in the context of a commitment to peaceful settlement. Chapter Ten is concerned with the legality of economic coercion, and also with whether the legality of that concept has a bearing on the question of lawful counter-measures. The final Chapter summarises the major characteristics of the existing legal regime of counter-measures.</p