Semiconductor diode lasers have significant advantages over other types of lasers in compactness and efficiency. They also can be mass produced using techniques of the electronics industry, offering the hope for very low cost per laser. However, the power of an individual single- spatial-mode diode laser is limited to a few watts. The coherent power of diode lasers can be increased by making a coherent array. We will discuss a variety of approaches that have been employed to make coherent arrays of diode lasers. The first group of techniques are those using a master oscillator, either in a branching master-oscillator-power- amplifier configurations, or in a multiple injection locked configuration. Systems without a master oscillator can either be one large resonator with many segmented gain regions, or coupled oscillators. The coupled oscillators are divided into those coupled to adjacent elements only (series coupling), and those coupled to many other elements (parallel coupling). We also will show how nonlinear optics have been used to implement some of the above architectures. These various methods have produced scientific success in coupling arrays together, but each has drawbacks that have prevented coherent diode arrays from achieving commercial success.