Ultrasonics, a branch of acoustics dealing with sound waves beyond the frequency range of human hearing, plays a crucial role in the non-destructive evaluation and characterization of materials at elevated temperatures. In materials science and engineering, ultrasonic techniques provide valuable insights into the internal structure, mechanical properties, structural transformations/phase transitions, and overall integrity of materials. High-temperature material’s characterization using in-situ ultrasonic parameter measurement is a valuable technique in materials science and engineering, and metallurgy. The temperature-dependent ultrasonic velocity and attenuation are the key parameters for the high-temperature material’s characterization. This approach involves monitoring the changes in ultrasonic velocity and attenuation of materials at elevated temperatures, providing insights into their mechanical and structural properties under desired conditions. The velocity of ultrasound measured as the function of temperature can be influenced by temperature-induced changes in the material’s properties. The changes in attenuation can reveal information about material microstructure and defects. In the pursuit of developing materials for high-temperature applications, understanding their behavior under systematic thermal conditions is paramount. The in-situ ultrasonic velocity and attenuation measurement emerge as a powerful tool, offering real-time insights into the structural and mechanical transformations materials undergo at elevated temperatures. This chapter covers the significance, advantages, experimental set-up, and key components of employing in-situ ultrasonic techniques for characterizing materials in high-temperature environments. This chapter also outlines the characterization of temperature-dependent phase/structural changes existing in the functional materials such as perovskite, ferrites, and bio-glasses.