Fusion product diagnostics can be used to determine a fusion reaction rate, which indicates how close the plasma is to the ultimate goal of making a power plant based on nuclear fusion. However, these diagnostics can also provide large amounts of additional information, such as ion temperatures, the thermonuclear fraction in the fusion reaction rate, degree of fast ion confinement, fast ion loss mechanism, etc. Measurement systems for fusion product diagnostics are usually designed and optimized to a specific performance so that they play different roles in the experiment. The neutron emission rate, which is directly related to the fusion output, can be determined by (a) time-resolved emission monitors, which are well calibrated onsite, in combination with (b) activation systems and (c) profile monitors with accuracy up to several percent. The time-resolved neutron profiles also provide useful information for transport analysis. Velocity distributions and confinement properties of fast ions can be obtained from (d) the neutron spectrometers and (e) gamma-ray measurement. The interaction between plasma dynamics and fast ions can be studied with most fusion product diagnostic systems, especially with (f) escaping charged fusion product detectors. Each section of this chapter contains a general explanation of these systems, showing some experimental results obtained on present devices. A lot of interesting and useful information on the behavior of energetic particles and their degree of confinement are provided by them because interaction between thermal and nonthermal energetic ions and that among nonthermal ions contribute dominantly to the fusion reaction rate in present deuterium-deuterium experiments. In future deuterium-tritium fusion experiments on ITER, the contribution of thermonuclear fraction will be increased, and the combination of several neutron measurement systems will provide the absolute fusion output and neutron fluence on the first wall. Together with neutron measurement, alpha particle and gamma-ray measurement play important roles in research on self-heating burning plasma physics and hence in the burning control of the device.