The imbibition of wetting liquids in porous media takes place on length scales spanning several orders of magnitude, in phenomena ranging from landslide due to heavy rainfall in geophysics to stiction of nanopatterns in semiconductor manufacturing. We review the current theoretical understanding of the dynamics of liquid infiltration that are driven by capillary forces and resisted by viscous friction. Physical principles that govern the flows in smooth channels and porous media, either with or without gravitational effects, are explained with simplified mathematical solutions. Also, some important but unexplored topics associated with capillary imbibition are suggested.