Quantum Cascade (QC) lasers are a rapidly evolving mid-infrared and THz, semiconductor laser technology based on intersubband transitions in multiple coupled quantum wells. The lasers’ strengths are their wavelength tailorability, high performance and fascinating design potential. We will first give a brief introduction into QC lasers followed by a discussion of several recent highlights, such as the quest for high performance QC lasers, especially high efficiency and single-mode operation, and the implementation of unconventional laser schemes. We will also discuss several applications, such as our field campaigns during the 2008 Beijing Olympics and in Ghana in 2009 and 2010. As an example for high-performance QC lasers, we examine lasers around 5 m wavelength. First, we focus on thorough engineering of conventional QC lasers. The quest for high power and high efficiency QC lasers requires these lasers to have a low intrinsic threshold, a high characteristic temperature, a low voltage defect, and superior heat sinking. QC lasers with several percent wall-plug efficiency at room temperature and few 10% efficiency at low temperatures are possible. Next, we move on to unconventional designs, and a recent innovation in how the carrier injection into QC laser active regions is described. The resultant QC lasers are nearly 50% power efficient at cryogenic temperatures. With respect to spectral innovations, a spectrally broadband QC laser based on a ‘continuum-to-continuum’ design will be presented, which differs from conventional, artificially spectrally broadened QC lasers in that almost no trade-off needs to be made between gain-bandwidth and laser performance with respect to laser threshold and output power. When this laser is put into an external cavity, a wide, continuous single-mode tuning range of well over 300 cm-1 is achieved. Next we explore opportunities for obtaining single-mode and tunable emission without the need of dispersive gratings, such as external dispersive cavities or gratings etched into the lasers.. Folded cavities, “candy-cane“-shaped lasers, and extreme multi-section lasers have all shown great potential for achieving single-mode emission at reduced fabrication complexity and cost. In summer 2008 we deployed two QC laser-based trace gas sensors for air-quality measurements in Beijing, China, one point-sensing and one open-path remote sensing instrument for the detection of common air pollutants. During the summers of 2009 and 2010, we deployed an improved version of the open-path sensing instrument in Elmina, a fishing village in Ghana, to detect air toxics from wood burning and fish smoking. The work presented is mostly supported by MIRTHE (NSF-ERC) with smaller contributions from other sources; the work has been conducted in collaboration with many valued colleagues in our own research group and across MIRTHE.