INTRODUCTION The skin barrier is fundamental to our existence, protecting us from dehydration and xenobiotics. Different techniques of skin barrier assessment are used, depending on whether the measurements are carried out in vivo or in vitro. For in vivo skin barrier assessment, the main technique is transepidermal water loss (TEWL) measurement, because it is safe and noninvasive. Of course, TEWL measurements only give information about the water barrier, whereas other chemicals will generally have different penetration properties. For in vitro barrier assessment, there are more methods available. This is because chemicals and formulations other than water can be used irrespective of toxicity, and because there is access to both sides of the barrier membrane. TEWL measurement may also have a role in this case because of the need, according to Organisation for Economic Co-operation and Development (OECD) guidelines (1,2), to verify the integrity of the membranes prior to the their use in penetration measurements. Tritiated water, electrical resistance, and TEWL procedures are recognized for such tests.