In developing the country, His Majesty has adhered to the natural principle, in which people employ natural resources available in each locality. The results are very satisfactory, for example, growing vetiver grass to prevent the erosion of top soil, as well as to conserve soil and water; improving deteriorated soil for natural farming; refores tation without planting; building check dams in the forests to conserve watershed areas; and using water hyacinth to solve the problem of polluted water. The simple operations of these projects have resulted in the balance of the entire ecological system, and led to sustainable development. The Royal Development Study Centres and various other projects have therefore been visited not only by practitioners but also by heads of s tate, prime ministers, as well as leaders of many countries. Eminent visitors include Her Majesty Queen Elizabeth II and Prince Philip, the Duke of Edinburgh, of the United Kingdom, Their Majesties King Carl XVI Gustav and Queen Sylvia of Sweden, His Excellency Nuhak Phumsawan, President of the Lao People’s Democratic Republic, and President Gloria Arroyo of the Republic of the Philippines.