Concatenating fields in field calculator of QGIS?

Is there a way to concatenate fields in field calculator of QGIS? (e.g. hectares + ' ha')

Answer

In recent QGIS versions (>= 2.6), + works for string concatenation

tostring(hectares) + ' ha'

Previously, the only concatenation operator was: ||

tostring(hectares) || ' ha'