Recently, erbium-doped fiber amplifiers (EDFA's) have become an indispensable element of fiber-optic transmission systems. The EDFA's are now being used to amplify the optical signal attenuated along the transmission fiber (instead of complicated optoelectronic regenerators), to improve the receiver sensitivities, and to boost the optical power before distribution. In many of these applications, noise figure (NF) is one of the most important characteristics which determines the performance of EDFA's. Several techniques have been proposed to measure the NF of EDFA's. In this thesis, the pros and cons of these techniques are compared experimentally. The NF's are measured by three different techniques in optical, time, electrical domain for various types of EDFA's. The results are compared with the NF obtained by the bit-error-rate measurement of amplified lightwave systems operating at 2.5 Gb/s. The simplest technique is to measure the NF in optical domain. Although this technique is widely accepted in industries, it may be inaccurate especially when the signal is contaminated with relative intensity noises. The technique to measure the NF in electrical domain using an RF spectrum analyzer provides the highest accuracy since it could account for all noises at the receiver.