This book contains four selected chapters on pyrimidines and their importance. The first chapter examines the synthesis, physico-chemical properties, and anti-fungal activity of new hybrids of thiazolo[4,5-D] pyrimidines with (1H-1,2, 4) triazole. The second chapter reviews pyrimidine ring containing natural products and their biological importance. The third chapter examines the regulation of the pyrimidine biosynthetic pathway in the bacterium Pseudomonas chlororaphis. The fourth and final chapter looks at pyrimidines as potential corrosion inhibitors.