This book systematically discusses the effects of static electric and magnetic and laser fields on the rotational, fine, and hyperfine structure of molecules and on interactions of molecules at sub-Kelvin temperatures. The theories of molecular interactions in the presence of external fields have been developed only recently, in order to address the experimental work with ultracold molecules. This text gives the first comprehensive discussion on intermolecular interactions in external fields.<p>Topics of discussion in this book include: rotational, fine and hyperfine structure of molecular radicals; 1Sigma diatomic molecules; 2Sigma and 3 Sigma molecules; 2Pi molecules; external field traps for ultracold molecules; interactions of ultracold molecules; Feshbach resonances in molecular scattering; molecular collisions in dc fields; molecular collisions in reduced geometries.</p>