In this talk we will explain what O-minimal geometry is and how it can be used to study variational Hodge theory. We will discuss what it means for a period map associated to a pure polarised integral VHS on a smooth complex quasi projective variety to be definable in a suitable O-minimal structure. Thanks to an O-minimal GAGA theorem, the definability of the period map implies Borel’s algebricity theorem as well as Cattani-Deligne-Kaplan’s algebricity of the Hodge loci. Both the O-minimal and the classical approach relay on the study of degenerations of Hodge structures (the SL_2^n orbit theorem) but we hope the O-minimal path will offer new insights into the aforementioned algebricity results.