Resonances in the photo-absorption spectrum of the generating medium can modify the spectrum of high order harmonics. In particular, window-type Fano resonances can reduce photo-absorption within a narrow spectral region and, consequently, lead to an enhanced emission of high-order harmonics in absorptionlimited generation conditions. For high harmonic generation in argon it is shown that the 3s3p 6 np 1 P 1 window resonances (n=4,5,6) give rise to enhanced photon yield. In particular, the 3s3p 6 4p 1 P 1 resonance at 26.6 eV allows a relative enhancement up to a factor of 30 compared to the characteristic photon emission of the neighboring harmonic order. This enhanced, spectrally isolated and coherent photon emission line has a relative energy bandwidth of only ΔE/E=3·10 -3 . Therefore, it might be directly applied for precision spectroscopy or coherent diffractive imaging without the need of additional spectral filtering. The presented mechanism can be employed for tailoring and controlling the high harmonic emission of manifold target materials. High harmonic generation (HHG) driven by a strong laser field represents an attractive method for generating coherent radiation in the extreme ultraviolet spectral region [1,2] and is, nowadays, widely employed in atomic, molecular, plasma and solid state physics. Recently, even keV photon energies have been demonstrated by HHG providing a bright source of coherent X-rays [3]. In addition, HHG allows producing extremely short attosecond pulses, which enable groundbreaking investigations in the field of attosecond science [4]. Typically, HHG suffers however from its inherently low conversion efficiency, which hinders applications that are especially dependent on a high photon flux in order to achieve a good signal-to-noise ratio.In a simple single atom picture HHG can be described by a three-step model [5]: First, an electron tunnel-ionizes through the atomic potential, which is modified by the strong laser field. Second, the electron propagates in the strong laser field. Finally, the electron may recombine with its parent ion and gives, thus, rise to the emission of a photon (third step). In this picture, the power emitted by a single atom at the frequency ω q is proportional to the square of the amplitude of the oscillating dipole |A q (t)| 2 , which is induced by the recombination process [5].While the response of a single atom to the laser field (A q ) is extremely small, phase matching of a large number of emitters gives rise to a coherent build-up of high-order harmonic (HH) emission along the propagation direction of the driving laser through the generation medium.By optimizing the macroscopic generation conditions, hence, maximizing the number of phase-matched emitters, conversion efficiencies as high as of 4·10 -4 at 73 nm (17 eV) [6] and 5·10 -5 at 53 nm (23.3 eV) [7] have been achieved with 800 nm driving wavelength. With the help of a simple model, which takes the emission of all emitters along the propagation axis, phase-mismatch and absorption into accou...