A theoretical study of the electronic structure of shallow and deep electron states caused by quantitative disorder, relaxed defects and impurity atoms in amorphous silicon has been made by using the cluster Bethe lattice method incorporating an LCAO (linear combination of atomic orbitals) basis set. Although localised states in the neighbourhood of the band edges of the fundamental gap arise from both the Si-Si bond length elongation and bond angle disorder, the effects of bond angle variance are more dominant. Similarly for the relaxed dangling bond, quite a small shift in the location of the state in the gap occurs by Si-Si bond length changes but significant shifts appear as a result of the variation in Si-Si bond angles. Thus, the state energy changes from 0.49 to 1.55 eV for a variation in the bond angle by -10% to +10%. Localised/resonance states possessing A₁ and/or T₂ symmetry appear throughout the band gap for all the interstitial atoms belonging to groups II to VI of the periodic table and for most of the substitutional atoms. Gap states are not seen for the small-sized substitutional atoms like Be and B. The electronegativity of an impurity does not seem to play any role in determining the location of the state in the fundamental gap. The present results are in good qualitative and semi-quantitative agreement with the experimental and self-consistent pseudopotential calculation results wherever they are available for either crystalline or amorphous silicon.