The rodent hippocampal formation constructs a spatial representation of the local environment which can be used to identify the animal’s current location, to remember events that happened there in the past, and to navigate to desirable locations in that environment. Spatial cells found in the hippocampal formation represent the animal’s location (place cells), its current heading direction (head direction cells), the metric of the environment (grid cells), and the animal’s distance from boundaries of the environment (boundary vector cells). These cells provide two ways of identifying the animal’s location, first on the basis of its relationship to environmental landmarks, and second on the basis of path integration signals which update this location on the basis of interoceptive cues from the vestibular, proprioceptive and motor systems. All of the cells use firing rate as the code for spatial representation. In addition, however place and grid cells use a timing code. This timing code takes the form of the phase of spike firing relative to the ongoing theta – local field potential (LFP) wave. The sinusoidal theta rhythm is a prominent feature of the hippocampal LFP which ranges between 6 – 11 Hz in the rat, the rate varying as a function of the animal’s running speed. We have suggested that theta-LFP is an integral part of one of the mechanisms by which the hippocampus carries out spatial computations. A key idea here is that there is not one but several theta -like oscillations of differing frequencies which produce oscillatory interference patterns which can account for many of the properties of place and grid cell firing. I will describe these ideas and discuss recent experiments which provide evidence in favour of and against them.