This is the accepted manuscript. The final version is available at http://www.biochemsoctrans.org/content/43/5/881. ; Studying protein folding and protein design in globular proteins presents significant challenges because of the two related features, topological complexity and cooperativity. In contrast, tandem-repeat proteins have regular and modular structures composed of linearly arrayed motifs. This means that the biophysics of even giant repeat proteins is highly amenable to dissection and to rational design. Here we discuss what has been learnt about the folding mechanisms of tandemrepeat proteins. The defining features that have emerged are: (i) accessibility of multiple distinct routes between denatured and native states, both at equilibrium and under kinetic conditions; (ii) different routes are favoured for folding versus unfolding; (iii) unfolding energy barriers are broad, reflecting stepwise unraveling of an array repeat by repeat; (iv) highly cooperative unfolding at equilibrium and the potential for exceptionally high thermodynamic stabilities by introducing consensus residues; (v) under force, helical-repeat structures are very weak with non-cooperative unfolding leading to elasticity and buffering effects. This level of understanding should enable us to create repeat proteins with made-to-measure folding mechanisms, in which one can dial into the sequence the order of repeat folding, number of pathways taken, step size (cooperativity) and fine-structure of the kinetic energy barriers. ; We acknowledge funding from the Medical Research Council of the UK (grant G1002329) and the Leverhulme Trust. AP is funded by a BBSRC Doctoral Training Program studentship. LSI acknowledges support of a Fellowship from the Medical Research Foundation.