Highly sensitive response of semiconducting single-walled carbon nanotubes (SWNTs) to molecular adsorption may lead to a unique direction in exploiting their exceptional electrical properties. For example, simultaneous doping and nearly ideal gate efficiencies are achieved with polymer electrolytes as gating medium for nanotube transistors. However, highly sensitive responses can also lead to difficulties in interpretation of many observations as exemplified by the controversy surrounding whether oxygen adsorption causes doping or changes in the nature of SWNT-metal contacts. Effects of molecular adsorption, both covalent and non-covalent, on the electronic properties of SWNTs are discussed. How electronically selective covalent chemistry changes Raman scattering and electrical conductivity of individual metallic and semiconducting as well as random networks of SWNTs are first discussed. Non-covalent adsorption of polymers is then explored where electrochemical gating can be applied to allow both chemical and electrostatic control of charge carriers.