In many applications it is important to be able to sample paths of SDEs conditional
on observations of various kinds. This paper studies SPDEs which solve such sampling problems.
The SPDE may be viewed as an infinite dimensional analogue of the Langevin SDE used in finite
dimensional sampling. Here the theory is developed for conditioned Gaussian processes for which
the resulting SPDE is linear. Applications include the Kalman-Bucy filter/smoother. A companion
paper studies the nonlinear case, building on the linear analysis provided here.