In this talk, I discuss an operator product expansion for planar form factors of local operators in N=4 SYM theory. In this expansion, a form factor is decomposed into a sequence of known pentagon transitions and a new universal object - the form factor transition. This transition is subject to a set of non-trivial bootstrap constraints, which can be used to determine it at finite coupling. I demonstrate this for MHV form factors of the chiral half of the stress tensor supermultiplet, which in particular contains the chiral Lagrangian and the 20'. ---- Part of London Integrability Journal Club. If you are a new participant, please register filling the form at integrability-london.weebly.com. The link will be emailed.