Mid-infrared fiber lasers have made significant strides in the last few decades and are now beginning to compete with and, in some cases, supersede traditional laser systems in terms of performance. In this chapter, we briefly review the current state of the field of lasers in the mid-infrared including traditional workhorse systems based on parametric amplification, transition metals, and quantum cascading. This is followed by a look at the materials science of mid-infrared compatible optical fiber and rare-earth doping in soft glasses. The advances in fiber laser sources operating near 3 μm are then explored including progress in narrow-linewidth and ultrafast systems. As an example of applications of these laser sources, we also present recent results in supercontinuum generation based on the 3 μm class fiber laser.