A quarter of a century after its (serendipitous) discovery, the catalytic sequential chaining of two carbon monoxides into an organic substrate (the so called double carbonylation reaction), is still not well understood. After a review of the most important experimental results, this Perspective proposes a critical survey of the main organometallic models and outlines some rather neglected tracks. Finally, the issue of the chaining of more than two carbon monoxides is also addressed. © The Royal Society of Chemistry 2003.