This review provides an update on the promises and pitfalls when using in vitro bioassays to evaluate beneficial and adverse health effects of botanicals and botanical preparations. Important issues addressed in the paper are: (i) the type of assays and biological effects available; (ii) false-positives, false-negatives and confounding factors; (iii) matrix and combination effects; (iv) extrapolation of in vitro data to the in vivo situation; (v) when (not) to use bioassays; and (vi) identification of active constituents. It is concluded that in vitro bioassays provide models to detect beneficial as well as adverse activities, but that linking these observations to individual ingredients and extrapolations to the in vivo situation is more complicated than generally anticipated. In vitro assays are widely and effectively used to test the efficacy and adverse effects of botanicals and botanical preparations. The pitfalls of these assays are however often incorrectly and inaccurately taken into account hampering adequate extrapolation to in vivo situations often resulting in false interpretations. These pitfalls as well as strategies to overcome them are discussed.