We propose two characterizations of complexity classes by means of programming languages. The first concerns Logspace while the second leads to Ptime. This latter characterization shows that adding a choice command to a Ptime language (the language WHILE of Jones) may not necessarily provide NPtime computations. The result is close to Cook who used “auxiliary push-down automata”. Logspace is obtained through a decidable mechanism of tiering. It is based on an analysis of deforestation due to Wadler in. We get also a characterization of NLogspace.