Most slope failures in tropical soils are triggered by local rainfall leading to changes in the soil-water conditions. The objective of this manual is to provide advice on how to determine if these conditions are critical and if so how to measure them. The predictions of stability are based on a combined slope hydrology/stability model developed at Bristol University under ODA sponsored research. The results have been tested under field conditions and integrated with existing practice.The main part of the manual consists of a series of 'Instructions', which are procedures to expand existing site investigations. These introduce new slope stability design charts, the use of the dimensionless resistance envelope and the measurement of permeability and suction together with factors affecting the use of piezometers. The manual concludes with a section on data collection.