  We introduce a prescription to define form factor integrands at loop level in
planar $\mathcal{N}=4$ supersymmetric Yang-Mills theory. This relies on a
periodic kinematic configuration that has been instrumental to describe form
factors at strong coupling in terms of periodic Wilson loops. With this
prescription, we are able to formulate loop-level recursion relations for
planar form factor integrands, using a two-line (BCFW) and an all-line shift.
We also point out important differences with the known recursion relations of
integrands of planar loop amplitudes. We present a number of concrete one-loop
examples to illustrate and validate our prescription for form factor
integrands.
