This is Volume XIX of a series of nineteen on Abnormal and Clinical Psychology. Originally published in 1932, this study looks at practical applications and bio-physiological foundations of Kretschmer's types. This work has two main aims: First: To apply the clinically originated constitution psychological typology of Kretschmer (with the extensions of Delbriick, Pfahler, etc.) to male juvenile delinquents. Secondly, to co-ordinate--as far as it was possible within the specialized framework--the bio-typologies of Kretschmer, Jaensch, Pende, Berman, Wiersma, etc., in so far as there appears to be similarity between the underlying biological principles of these typologies and those of Kretschmer's typology.