Over the last 10 years, there has been a revolutionary change in the resistivity (and IP) surveying method. Until the early 1990s, the resistivity method has been used as a one-dimensional (1D) tool where quantitative modelling was confined to simple horizontal layers which are not sufficiently accurate for complex geological environments. However due to recent developments in field equipment design, interpretation software and microcomputer technology, two-dimensional (2D) and even three-dimensional (3D) surveys are now practical geophysical exploration tools for environmental and engineering studies. RES3DINV is the 3D resistivity and IP inversion software, and it is probably the first widely available commercial software of this type. 3D surveys are more time-consuming and expensive to carry out, but could be useful for very complex geological situations. It supports surveys with up to about 6240 electrodes and 75000 data points. In addition to surveys with a traditional rectangular grid of electrodes, it also supports surveys with trapezoidal grids which is common in rugged and/or forested areas. It now supports parallel calculations on Pentium 3 and 4 CPUs that significantly reduces the inversion time for very large survey grids. On a modern Pentiun 4 based microcomputer, the data inversion takes less than a minute for small surveys with 100 electrodes in a flat area, to a day for extermely large surveys with with thousands of electrodes in rugged terrain.