This chapter outlines the concept and definition of functional polymers with some appropriate examples. Functional polymers are defined as polymers with mechanical, chemical, physical, or optical properties or characteristics imparted by introducing molecular moieties with specific functions or properties into main chains, side chains, or both main and side chains. This chapter provides an overview focusing on a group of polymers with electrical and electronic properties, magnetic properties, optical and chiroptical properties, and liquid crystal properties. The functionalities of polymers are classified into the following categories: mechanical properties (elastic modulus, viscosity, tensile strength, etc.); electrical properties (conductivity, resistivity, dielectric constant, etc.); electro-optical properties (electroluminescence, photovoltaics, etc.); photoelectrical properties (photovoltaics, etc.); thermoelectric properties (thermoelectricity, thermopower, etc.); magnetic properties (paramagnetism, ferromagnetism, solenoid magnetism, etc.); photo responsiveness (optical responsiveness, stimulus responsiveness, fatigue resistance, etc.); mesogenic properties (thermotropic liquid crystal, lyotropic liquid crystal, etc.); ferroelectric properties (ferroelectric memory, spontaneous and residual polarization, capacitors, etc.); and helical properties (self-assembled whisker, birefringence, circular dichroism, linearly polarized and circularly polarized luminescence, etc.). These functionalities can be achieved or modulated by changing the molecular design of polymers, molecular moieties of repeating units, and polymerization conditions to achieve the intended properties and characteristics.