1 Introduction  
2 Observed source counts  
2.2 Low frequency surveys  
2.3 Deep surveys and sub-mJy counts  
2.4 High frequency surveys and counts  
3 Local luminosity functions  
4 Source spectra and evolution  
5 Evolutionary models: radio AGNs  
5.1 Low versus high luminosities  
5.2 Steep- versus flat-spectrum sources  
5.3 High $z$ evolution  
5.5 Special classes of sources  
6 Evolutionary models: star-forming galaxies  
6.1 Star-forming, normal and sub-mm galaxies  
7 The radio background  
8 Sunyaev-Zeldovich effects  
8.2 Galaxy-scale Sunyaev-Zeldovich effects  
9 Wide area surveys and large scale structure  
9.1 The angular correlation function and its implications  
10 The future  
11 Conclusions