The aim of this paper is to discuss some basic experiments on mixing by turbulent flows, which give a clear understanding of the main mechanisms entering into the mixing process. A very small number of such fundamental physical situations may be found in the literature, from the simplest one concerning mixing in homogeneous turbulence up to more complex situations such as those encountered in mixing layers, which are strongly inhomogeneous. These experimental results enable some definite conclusions to be drawn about basic average mechanisms such as the transfer from large to small scales of the concentration field, and about some scaling laws derived from the understanding of these mechanisms. Nevertheless, the delicate problem of the topology of the finest scales of the concentration field remains open particularly for liquids where the Schmidt number is high, and, consequently these scales are very small. Only very recent experimental work, using image analysis, gives some answers to this question, which is essential for the modeling of molecular diffusion processes. The different theoretical results from turbulence theory needed for the discussion will be reviewed in this article, without any straight forward developments, but with their physical meaning.