Data quality, especially in underwater scenarios, is the ultimate factor in deciding the success or failure of the measurement system. Transducers are performing out of their intended specification requirements, leading to false data being measured. Data quality must be established categorically before it is used for any analysis application. The calibration is fundamental for accurate measurements and to estimate the characteristics of hydrophones. Calibration establishes the relationship between the measured values and those realized by the reference standard under specified conditions. This chapter outlines the methods used for underwater electroacoustic transducer/hydrophone calibration. It is also describing about the underwater acoustic terminologies and preconditioning of transducers before being calibrated. The sources of uncertainties pertaining to each method are also explained in this chapter. Absolute uncertainty can never be achieved, and hence the uncertainty range is estimated with a traceability chain with a reference to national or international standards. The procedure described here is on the basis of the IEC 60565 standard and the basis of any quality assurance program and complements the essential requirements for accreditation.