This is a brief overview of the lessons learnt from BRE technical consultancy case studies involving the deterioration of cement-based building materials and components. The wide range of damaging reactions included are: sulfate reactions (thaumasite and delayed ettringite formation), steel-slag expansive reactions (in fill and industrial waste deposits), and acid attack on concrete. Recommendations on how to avoid damage are given. A common characteristic of a large proportion of the cases studied was the build-up of moisture, which was necessary for the damaging reactions to occur. A particular cause for concern is where measures have been deliberately taken to prevent moisture gaining access, but these have actually caused a moisture build-up by preventing the moisture that has collected from escaping, eg the use of render or impermeable membranes. 6 pages.