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Quantum Cascade Lasers (QCLs) are semiconductor sources based on inter sub-band transitions in quantum wells. This concept has permitted powerful and compact laser sources covering the entire mid-infrared (MIR) and terahertz spectral regions. Recently, it has been demonstrated that QCLs can operate as frequency combs (FCs), which holds an exciting perspective of high resolution and high sensitivity spectrometers with no moving parts. To realise FCs over a large part of the optical spectrum, dispersion compensation needs to be considered (i.e. for equidistant mode spacing). However, current techniques have not been incompatible for the generation of output high powers owing to strong optical absorption of dielectric coatings that have been used. There have been considerable advances to overcome these bottlenecks.