  We review theoretical approaches, experiments and numerical simulations that
have been recently proposed to investigate the folding problem in single-domain
proteins. From a theoretical point of view, we emphasize the energy landscape
approach. As far as experiments are concerned, we focus on the recent
development of single-molecule techniques. In particular, we compare the
results obtained with two main techniques: single protein force measurements
with optical tweezers and single-molecule fluorescence in studies on the same
protein (RNase H). This allows us to point out some controversial issues such
as the nature of the denatured and intermediate states and possible folding
pathways. After reviewing the various numerical simulation techniques, we show
that on-lattice protein-like models can help to understand many controversial
issues.
