This thesis aims at bringing support to the general thesis of essentialism about modality: all necessity is fully grounded in essence, and possibility is defined accordingly. First, it motivates essentialism about metaphysical necessity (including logical, conceptual and mathematical necessities as special cases of it). Second, and most importantly, it defends an essentialist account of natural modality. On this account, in particular, natural necessity (e.g. causal laws of nature) is homogeneous, absolute, unconditional, and fully grounded in the essences of natural entities (kinds, properties, objects). Third, relying on those bases, it suggests a perfectly homogeneous essentialist account of 'absolute' modality, and then considers how essentialism may be extended to all of modality, including non-abolute kinds of modality.