The introduction of the exact 
<br>couple 
<br>The introduction of the spectral sequence 
<br>The Serre spectral sequence
<br>The Atiyah-Hirbruch spectral 
<br>sequence
<br>The Adams spectral sequence
<br>The Brown-Peterson theory 
<br>The Adams-Novikov spectral 
<br>sequence
<br> 
<br>