The value and power of economic big data is increasingly being recognised as an important asset of states in their endeavours on competitive and globalised economic markets. Against this background various methods to gather massive amounts of secret, publicly unavailable economic information from third states are being used. In this respect, cyber space has become an ultimate tool enabling relatively easy, sophisticated and quick access to large amounts of confidential information, essential for the performance and operation of businesses and the economic stability of states. This chapter focuses on economic cyber espionage among states and assesses the legality of such activities under international law. Emphasis is on three international legal frameworks: bilateral agreements between states, general international law rules on non-intervention and trade policy tools. This chapter argues that economically motivated cyber espionage activities by states and their status under international law should be differentiated from other forms of traditional espionage conducted for military, strategic and security reasons. While the legality of traditional espionage activities at the international level remains uncertain, we are witnessing important developments in the area of economic cyber espionage, which received significant attention in inter-state economic relations, thereby indicating a need for further international regulation.