On the theory of unipotent algebraic groups over an arbitrary ground field -- Notations, conventions and some basic preliminery facts -- Forms of vector groups; groups of Russell type -- Decomposition theorems for central extensions of co utative group schemes; application to the two-dimensional unipotent groups -- Wound unipotent groups -- The question of commutativity for two-dimensional unipotent groups -- Forms of the affine line and geometry of the groups of Russell type -- Actions of unipotent group schemes -- The underlying scheme of a unipotent algebraic group -- The hyperalgebra of a unipotent group scheme.