Aluminium in solution in the ferrite in ferrite/pearlite steels has been found to have little influence on strength of the ferrite probably because up-till recently it has been added in very small amounts (0˙02-0˙08 wt-%Al) as a grain refining addition. There is indeed some doubt in the literature as to whether Al can solid solution harden. This lack of knowledge has not been important while Al additions are small but recently Al has been added in much greater amounts 0˙5-2% in some new grades of transformation induced plasticity (TRIP) steels. These steels have often an essentially ferritic structure with retained austenite and bainitic ferrite. The main thrust of the paper has therefore been to determine the solid solution hardening effect of Al in ferritic structures. Steels with varying Al levels from 0˙02 to ∼2% at two C levels, 0˙02 and 0˙1% were tensile tested after hot rolling so as to determine the solid solution hardening effect of Al. It was found in the 0˙1%C steel that some of the pearlite transformed to martensite on raising the Al level to 1% and this resulted in preyielding and lower strength levels than would have been obtained from a ferrite/pearlite structure. However, when the Al level was kept to the maximum allowable before martensite formed a marked solid solution hardening by Al was observed. Hall-Petch plots were also obtained from steels in which martensite was not present and it was found that a 1%Al addition increased the yield strength by 70-80 MPa and this increase was not dependent on the grain size as has been found for Si. It can be concluded that Al is a substantial solid solution hardener of steel but the strengthening effect of Al is often obscured at low additions by its ability to remove nitrogen from solution and at high additions by its tendency to favour martensite formation and cause preyielding of the ferrite