This chapter presents an introduction to acoustics and explains the basic quantities and concepts relevant to terrestrial and aquatic animal bioacoustics. Specific terminology that is introduced includes sound pressure, sound exposure, particle velocity, sound speed, longitudinal and transverse waves, frequency-modulation, amplitude-modulation, decibel, source level, near-field, far-field, frequency weighting, power spectral density, and one-third octave band level, amongst others. The chapter then introduces basic signal sampling and processing concepts such as sampling frequency, Nyquist frequency, aliasing, windowing, and Fourier transform. The chapter concludes with an introductory treatise of sound localization and tracking, including time difference of arrival and beamforming.