  T2K is a long-baseline neutrino oscillation experiment searching for $\nu_e$
appearance in a $\nu_{\mu}$ beam. The beam is produced at the J-PARC
accelerator complex in Tokai, Japan, and the neutrinos are detected by the
Super-Kamiokande detector located 295 km away in Kamioka. A suite of near
detectors (ND280) located 280 m downstream of the production target is used to
characterize the components of the beam before they have had a chance to
oscillate and to better understand various neutrino interactions on several
nuclei. This paper describes the design and construction of two massive
fine-grained detectors (FGDs) that serve as active targets in the ND280
tracker. One FGD is composed solely of scintillator bars while the other is
partly scintillator and partly water. Each element of the FGDs is described,
including the wavelength shifting fiber and Multi-Pixel Photon Counter used to
collect the light signals, the readout electronics, and the calibration system.
Initial tests and {\normalsize\em in situ} results of the FGDs' performance are
also presented.
