Ohmic contacts with very low specific contact resistance are required for many electronic and optoelectronic devices. If the resistance is not low enough, the contacts can severely limit the performance of high frequency transistors and other devices. Additional requirements include uniformity on a length scale appropriate for the device, process compatibility, and reliability. In this seminar, approaches for the formation of low-resistance ohmic contacts are discussed. Special attention will be given to contacts to compound semiconductors and semiconductor nanowires, including strategies to dope the semiconductors as part of an annealing step, and the role of size and orientation on the reaction of metals with semiconductor nanowires and fins.