The ordered arrangement of pores made possible by crystalline framework materials has lead to exciting applications in gas storage, separation and catalysis. In particular, the crystalline engineering of porous materials constantly produces unique pore structures with pore sizes ranging from 2 to 40 Å. The ability to engineer a porous material with a specific pore size is key to meaningful performance in promising applications. However, a single pore environment is limited in application to a narrow regime of size and shape selectivity for guest molecules, often restricting mass transfer. Materials featuring hierarchical porosity can overcome this limitation providing an optimisation pathway for both increased diffusion and confinement, by combining different porosity scales. For example, fluid catalytic cracking catalysts used in industry commonly employ a hierarchical structure such as a USY zeolite that has mesoporosity mixed with a macroporous matrix. The advantages of combining several porosity regimes in a single material have been demonstrated in several applications relevant to the fields of catalysis and phase separation, however, aspects of guest behaviour in these porous systems remains uncertain. Discovering the mechanisms ruling adsorption in hierarchical solids is crucial for the future design of porous materials with optimal properties toward applications. Metal-organic frameworks (MOFs) are widely recognised as an important class of porous materials, featuring high porosity from well-defined and tailored structures at the atomic level. Many MOFs demonstrate hierarchical porosity and the sheer structural diversity of this materials class can enable unparalleled performance in adsorption applications. The clearly defined structure of MOFs has lead to unparalleled understanding of pore filling and other complex adsorption phenomena, such as breathing and negative gas adsorption. Gas adsorption is an essential characterisation tool for porous materials and can be coupled with X-ray diffraction to experimentally probe gas adsorption behaviour on a porous host. This in situ characterisation is widely employed to monitor the structure transition of flexible MOF crystals during adsorption. Moreover, by careful analysis, the average location of adsorbates within the pore structure can also be observed.