The book deals with the secession/separation of territorial entities and the legal consequences that derive from it both for the parent state and for the seceded/separated entity or the entity that intends to secede/separate. This subject is approached from the triple perspective of international law, comparative law, and Spanish law.International law, because it is this legal system which contains the general legal framework within which this issue must be dealt with. Thus, for example, the legal basis of the right to self-determination, the constituent elements of the state, the recognition of states and governments, succession in the matter of treaties, succession in membership of International Organisations, etc.Moreover, international law is also the reference invoked by secessionist/independentist political projects within states in an attempt to provide a legal basis for the legality of their claim.Comparative law, in order to find out not only how most state constitutions deal with secession or independence of their territories, but also the jurisprudence handed down by national courts on the matter (USA, Canada, Italy, France, Spain, among others).And finally, Spanish Law, because the perspective chosen to address the object of study is from the perspective of Spain.