The inception of excited-state reactivity paradigms that overcome thermodynamic constraints in the groundstate have been instrumental in advancing contemporary organic synthesis. Here, light can be harnessed to facilitate the efficient photochemical or photocatalytic activation of low energy abundant feedstock chemicals to construct higher in energy, strained complex chemical systems. In this regard, the light induced 2+2 cycloaddition of alkenes to form cyclobutanes, is perhaps most renown (Figure ), enabling the facile translation from 2D to 3D chemical space, constructing up to four contiguous stereocentres in a single process. Despite these advances, many intractable challenges that plague intermolecular reactivity, persist, often leading to uncontrolled reactivity/selectivity (Figure , bottom). These challenges include, competing homodimerization due to self 2+2 cycloaddition, formation of regioisomers as a result of differences in alkene approach, and variation in diastereoselectivity attributable to formed radical intermediates. Pioneering research has made notable strides in addressing such limitations enabling catalysis via non-covalent interactions or substrate control. Lewis acids or Brønsted acids have been leveraged, leading to concomitant enantioinduction, while advances in photocatalyst design has seen the inception of quantum dots, and chiral frameworks, to provide a wide platform of compatible precursors that are operational.