This study is primarily concerned with the cyanobacteria and algae growing on the exposed surfaces of buildings constructed of man-made materials. Special consideration is given to those obscuring coloured stains on building stone. Besides discussing the taxonomy, biology and ecology of the cyanóbacteria and algae concerned this account includes the tasha caused by them and the method used for their investigation, control and elimination. A lot of information is diffused in the literature with some in obscure reports which are not noted.