Monoclinic potassium lutetium double tungstate KLuW is established as new laser host for Tm, Yb and Ho active ions due to (i) its very high values of absorption and emission cross sections (ii) its relatively large Lu-Lu distances allowing high ion concentration with minimum fluorescence quenching effects and (iii) the long lifetime of the upper laser level ensuring high energy storage. These characteristics are exploited in this work using Tm as active ion to generate laser emission at 1.94 µm. The local maximum of water absorption at this wavelength makes this laser interesting for medical and atmospheric applications. Polarization switching, dual-wavelength operation, vibronic laser, thin-disk laser design, passive Q-switching and the laser-tissue interaction of the 1.94 µm Tm:KLuW laser are demonstrated in this book.