In this chapter we describe the application of in-cell NMR spectroscopy to the investigation of G-quadruplex structures inside living Xenopus laevis oocytes and in X. laevis egg extract. First, in-cell NMR spectroscopy of nucleic acids (NA) is introduced and applications and limitations of the approach are discussed. In the following text the application of in-cell NMR spectroscopy to investigation of G-quadruplexes are reviewed. Special emphasis is given to the discussion of the influence of the intracellular environmental factors such as low molecular weight compounds, molecular crowding, and hydration on structural behavior of G-quadruplexes. Finally, future perspectives of in-cell NMR spectroscopy for quantitative characterization of G-quadruplexes and NA are discussed.