The progress on multi-wavelength quantum cascade laser arrays in the mid-infrared is reviewed, which are a powerful, robust and versatile source for next-generation spectroscopy and stand-off detection systems. Various approaches for the array elements are discussed, from conventional distributed-feedback lasers over master-oscillator power-amplifier devices to tapered oscillators, and the performances of the different array types are compared. The challenges associated with reliably achieving single-mode operation at deterministic wavelengths for each laser element in combination with a uniform distribution of high output power across the array are discussed. An overview of the range of applications benefiting from the quantum cascade laser approach is given. The distinct and crucial advantages of arrays over external cavity quantum cascade lasers as tunable single-mode sources in the mid-infrared are discussed. Spectroscopy and hyperspectral imaging demonstrations by quantum cascade laser arrays are reviewed.