This article deals with the key issue of implementation of foreign law by courts, the allocation of duty to determine the foreign law substance between parties and court in particular. The article studies the common principle of application of foreign law and different approaches to foreign law in various legal systems. It is noted that such different approaches reflect with allocation of duties to prove foreign law between court and parties. In common law countries the primary duty to prove foreign law lays on parties, in continental law countries it lays on court.

The attention is focused on the fact that there is a tendency to minimize the differences in the approaches to foreign law in different legal systems, and this, in turn, affects the unification of the legal allocation of duties to determine its substance between parties and court.