An unsaturated soil mechanics approach was used to define critical state relations for agricultural soils. The modified cam clay model, an elasto- plastic constitutive model based on the critical state concept was used to model the soil behaviour. Mechanical properties of the hard setting soils thought to be pertinent to tillage were identified and constitutive relations developed through the critical state theories. The book is useful to researchers in the areas of critical state soil mechanics, mechanical behaviour of soils, soil tillage, and establishment of tillage machinery requirement and tool design. The book is highly relevant as reference material for engineering students in institutions of higher learning in; soil mechanics, tillage, machinery and modelling. Final year undergraduate students will find the book pertinent in Geotechnical design projects. Post graduate geotechnical students will find the book resourceful during their literature search and research reference. Practicing engineers in Geotechnical engineering, soil tillage and field machinery selection will find it relevant as a reference text.