The MOHID Water Modelling System is integrated water modelling software that can be used to simulate the dynamics of water bodies, porous media flow and infiltration, and watersheds. Over the past years MOHID Water has been used to simulate a variety of physical, chemical and ecological processes at different scales in marine systems. MOHID Water is the latest version of MOHID long set of evolutions which started back in 1985. Since then, a continuous development effort of new features has been maintained. Model updates and improvements have been made available on a regular basis and used in the framework of many research and engineering projects. Initially, MOHID was a two-dimensional tidal model written in FORTRAN 77. This version also gave the present name to the model, which derives from the Portuguese abbreviation of MOdelo HIDrodinâmico (Hydrodynamic Model). Traditionally known as a hydrodynamic model, it was first used to study estuaries and coastal areas using a classic finite-differences approach. Further developments included a 3D setup and addition of baroclinic effects, and full discretization to finite volumes approach allowing the use of generic vertical coordinates. In 2000 the development of MOHID LAND started with a 1D-Soil model solving Richards and van-Genuchten equations, which was expanded to include remaining watershed processes (aquifer, river, overland flow, vegetation growth, etc). A substantial increase in the number of users has occurred since the model was made available on the internet, backed up by an online user forum. Model robustness in hydrodynamics set the basis to the development and coupling of a transport model, including fine sediment transport. This development also allowed the coupling of a water quality (eutrophication) module, which increased the variety of model applications and transformed the model into a fully integrated tool. In time, the increase of MOHID programmers and users proved to be unsustainable due to the multidisciplinary nature of the endeavour and to FORTRAN 77 language limitations. The model was restructure and converted to ANSI FORTRAN 95, profiting from its new features such as the ability to produce object oriented programming.