This chapter details the frequency-domain properties of observational data on stars. It is from the analysis of the frequency spectrum of the observations that the asteroseismic parameters are usually extracted. The chapter looks at each of the intrinsic stellar contributions in turn, always with an eye to how each must be handled in the asteroseismic analysis. It also considers the impact on the analysis of gaps in the data. In performing the analyses, this chapter begins by covering practical fundamentals associated with estimation of the frequency spectra, which underpin everything that follows. In addition, this chapter will be making use of Fourier transforms in studying the data.