A comprehensive overview of seismic signals, waves, instrumentation, and their diverse applications is discussed in this chapter. Natural seismic signals including tectonic earthquakes, volcanic eruptions, landslides, and glacial seismic activity are discussed alongside artificial seismic signals such as collapse earthquakes, excavation-induced signals, and those generated by scientific experiments. The characteristics of seismic waves are examined, focusing on body waves (P and S waves) and surface waves (Love and Rayleigh waves), elucidating their nature and properties emanating from earthquakes. Seismic instrumentation is thoroughly explored, encompassing various sensors like seismometers, accelerometers, geophones, etc. The working principles of different seismometers including broadband, long-period, short-period, borehole, surface, and ocean-bottom variants are detailed, alongside discussions on geophones and the frequency responses of different sensors. A comparative analysis of seismic sensor manufacturers is presented in a tabular format. The components of seismic recorders, including signal sensing, data acquisition, time stamping, power supply, seismic trigger techniques, viz. amplitude and frequency-based triggers, STA over LTA, etc., and communication interfacing, are delineated, with a detailed examination of data formats associated with seismic signals. The chapter delves into the myriad applications of seismic signals and instrumentation, ranging from structural health monitoring to earthquake early warning systems, border intrusion detection, and animal movement tracking for human-animal conflict avoidance. Various seismic research software tools are discussed, underscoring their significance in seismic analysis and research. A brief overview of moonquakes, the cause and types of moonquakes, and the discoveries made by space agencies around the world are presented, adding an extra-terrestrial dimension to the seismic discourse. This comprehensive examination serves as a valuable resource for researchers, engineers, and practitioners involved in seismic studies and applications across diverse fields.