Since their discovery in the late 1960s, 1-azabicyclo[1.1.0]butanes have demonstrated to be interesting precursors of azetidines, because of the peculiar reactivity of the C3–N bond that allows double functionalization in the 1,3 positions. In particular, the recent advances reported by Baran, Lopchuk, Aggarwal, and others witness the synthetic relevance of such strained azabicycles in the synthesis of highly functionalized azetidines. However, the synthesis and reactivity of 1-azabicyclo[1.1.0]butanes remains a poorly explored topic in organic chemistry. This review aims to furnish a comprehensive knowledge on the preparation of 1-azabicyclo[1.1.0]butanes and the transformation into functionalized saturated four-membered azacycles.