New methods using different physical principles have been successfully applied in studies of root systems of large trees. The ground-penetrating radar technique provides 3D images of coarse roots (starting with a diameter of about 20 mm) from the soil surface down to a depth of several metres. This can even be done under layers of undisturbed materials such as concrete, asphalt and water. Fine roots cannot be visualized by this method, but the total rooted volume of soil can be determined. The differential electric conductance method has been used for fast measurement of conducting (absorbing) root surfaces. However, more testing is needed. Both these methods are non-invasive. The results can be verified by an almost harmless excavation of whole root systems, including fine roots, using the ultrasonic air-stream (air-spade) method. This method is suitable for all studies, as well as practical operations on roots or objects in their vicinity, where a gentle approach is required. Sap flow measurements on their own or in tandem with soil moisture monitoring play a leading role in studying root function and hydraulic redistribution of flow in the soil. The water absorption function of roots can be studied by measuring sap flow on individual root branches directly (as on crown branches) and also indirectly, by measuring the radial pattern of sap flow in different sapwood depths at the base of a stem. Root zone architecture can also be estimated indirectly by studying its functionality. The heat field deformation method with multi-point sensors has been found to be very convenient for this purpose. A combination of several such methods is recommended whenever possible, in order to obtain detailed information about the root systems of trees.