This book reported the literature survey since the pyrazolone chemistry began in 1883 by Ludwig Knorr to date concerning on tautomerism, synthetic approaches and chemical properties of title compounds as well as their chemical reactions. Furthermore, the obtained results were discussed, novel 2-Pyrazolin-5-ones were synthesized, the synthetic mechanisms were investigated and compounds were established by elemental analysis and spectral data. Additionally, these series of the synthesized dyes were applied as disperse dyes for dyeing polyester fabrics and their fastness properties were evaluated. Also the position of color in CIELAB coordinates (L*, a*, b*, H*, C*) was assessed.