  As said by Sir A. Eddington in 1925: Our telescopes may probe farther and
farther into the depths of space. At first sight it would seem that the deep
interior of the sun and stars is less accessible to scientific investigation
than any other region of the universe. What appliance can pierce through the
outer layers of a star and test the conditions within? Eddington (1926).
Nowadays, asteroseismology has proven its ability to pierce below stellar pho-
tospheres and allow us to see inside the interior of thousands of stars down to
the stellar cores, answering the question asked by Eddington ninety years ago.
In this chapter we review the general properties of the spectral analysis which
is the base of any asteroseismic investigation. After describing the stellar
power spectrum, we will describe in details the characterization of the modal
spec- trum. This chapter will end by a brief description of the instrumentation
in both helio and asteroseismology.
