The synthesis of unconventional extended π-systems is described in an attempt to tailor the structures of organic compounds for specific optical and electrical properties. In order to emphasize the role of the π-conjugation and to correlate chemical structure and physical function both one-dimensional arylenevinylenes and two-dimensional ribbon-type molecules are considered. In the synthesis of the former the aryl-olefin coupling according to Heck is of special value, the synthesis of the latter is achieved by repetitive Diels-Alder cycloadditions and by two-step processes in which carefully designed polyaryl precursors are subjected to ring closure. Key ingredients of the present approach are the interplay of synthetic organic chemistry and synthetic macromolecular chemistry and the needs outlined by materials sciences. Thereby, transition from monomers to oligomers and polymers defines new requirements for the selectivity of the synthetic reactions and the tractability of the products. 1. Introduction 2. Oligo- and Poly(arylenevinylene)s 2.1. Structurally Defined Oligo(1,4-phenylenevinylene)s - Valuable Model Systems for Understanding Properties of the Corresponding Polymers 2.2. Oligomers and Polymers Containing the 9,10-Anthrylene Unit in the Main Chain 2.3. Unusual Redoxactive Systems via Incorporation of 1,5-Cyclooctatetraenylene Units 2.4. The 2,2′-Biphenylylene Building Block - toward the Generation of Photoreactive Poly(arylenevinylene)s 3. π-Systems with Ribbon- or Ladder-Type Structures 3.1. Ribbon-Type Structures via Repetitive Diels-Alder reactions 3.2. Ribbon-Type Structures via Polymer-Analogous Cyclization of Linear Precursors 3.2.1. Double-Stranded Poly(phenylene)s via Cyclization of Open-Chain Precursors 3.2.2. Oligorylenes - Synthesis of Ribbon-Type Molecules with peri-Fused Naphthalene Units 4. Conclusions