ConspectusPolycyclic (hetero)aromatic hydrocarbons (PAHs) have emerged as a focal point in current interdisciplinary research, spanning the realms of chemistry, physics, and materials science. Possessing distinctive optical, electronic, and magnetic properties, these π-functional materials exhibit significant potential across diverse applications, including molecular electronic devices, organic spintronics, and biomedical functions, among others. Despite the extensive documentation of various PAHs over the decades, the efficient and precise synthesis of π-extended PAHs remains a formidable challenge, hindering their broader application. This challenge is primarily attributed to the intricate and often elusive nature of their synthesis, compounded by issues related to low solubility and unfavored stability.The development of π-building blocks that can be facilely and modularly transformed into diverse π-frameworks constitutes a potent strategy for the creation of novel PAH materials. For instance, based on the classic perylene diimide (PDI) unit, researchers such as Würthner, Wang, and Nuckolls have successfully synthesized a plethora of structurally diverse PAHs, as well as numerous other π-functional materials. However, until now the availability of such versatile building blocks is still severely limited, especially for those simultaneously having a facile preparation process, adequate solubilizing groups, favored material stability, and critically, rich possibilities for structural extension spaces.In this Account, we present an overview of our invention of a highly versatile bay-/ortho-octa-substituted perylene building block, designated as Per-4Br, for the construction of a series of novel PAH scaffolds with tailor-made structures and rich optoelectronic and magnetic properties. First, starting with a brief discussion of current challenges associated with the bottom-up synthesis of π-extended PAHs, we rationalize the key features of Per-4Br that enable facile access to new PAH molecules including its ease of large-scale preparation, favored material stability and solubility, and multiple flexible reaction sites, with a comparison to the PDI motif. Then, we showcase our rational design and sophisticated synthesis of a body of neutral or charged, closed- or open-shell, curved, or planar PAHs via controlled annulative π-extensions in different directions such as peripheral, diagonal, or multiple dimensions of the Per-4Br skeleton. In this part, the fundamental structure-property relationships between molecular conformations, electronic structures, and self-assembly behaviors of these PAHs and their unique physiochemical properties such as unusual open-shell ground states, global aromaticity, state-associated/stimuli-responsive magnetic activity, and charge transport characteristics will be emphatically elaborated. Finally, we offer our perspective on the continued advancement of π-functional materials based on Per-4Br, which, we posit, may stimulate heightened research interest in the versatile structural motifs typified by Per-4Br, consequently catalyzing further progress in the realm of organic π-functional materials.