Common Article 1 (common to the Geneva Conventions of August 1949 and their Additional Protocols I and III), provides that ‘[t]he High Contracting Parties undertake to respect and to ensure respect for the […] Convention [and Protocol] in all circumstances’ (CA1). This chapter introduces the key discussions around the meaning and significance of this provision looking at State and judicial practice pertaining to this provision over the last 50 years. Scholarship around this article, which has been touched on by numerous authors over the last 20 years, is considered. The chapter sets up the discussion that follows through the volume, across a range of areas of international humanitarian law (IHL), about what implementing CA1 might look like in practice.