Theoretical developments have gone hand in hand with developments in experimental methods , where the use of different forms of spectroscopy , such as infrared spectroscopy , microwave spectroscopy , EPR spectroscopy and NMR spectroscopy , is probably the most important 20th century development .
New forms of spectroscopy started in the 20th century including : infrared spectroscopy , microwave spectroscopy , EPR spectroscopy and NMR spectroscopy .