## 1 Introduction  
Virtually all plant species possess some kind of hair-like epidermal structures. When these structures are present on the aerial parts of a plant, they are commonly referred to as trichomes, while similar outgrowths from the root are called root hairs. Trichomes--the term deriving from the Greek word \"trichos\", which means hair--are, in most cases, not connected to the vascular system of the plant, but instead are extensions of the epidermis from which they originate [1]. Trichomes range in size from a few microns to several centimeters and they exhibit a tremendous species-specific diversity in shape (for examples, see [2]), and, therefore, they are often used as diagnostic characteristics for the identification of plant species, e.g., [3]. Trichomes are mainly found on leaves and stems, but they can also occur, depending on the species, on petals, petioles, peduncles and seeds [1]. Trichomes can be single-celled or multicellular, but the criterion that is mostly used to classify them is whether they are glandular or not [4]. Non-glandular trichomes are present on most angiosperms, but also on some gymnosperms and bryophytes [1]. On the model plant _Arabidopsis_, only non-glandular trichomes can be found, which are unicellular and can be either unbranched, or have two to five branches [5]. These trichomes are polyploid [6] and have been extensively studied with respect to their development, e.g., [7]. In contrast, glandular trichomes are usually multicellular, consisting of differentiated basal, stalk and apical cells and can be found on approximately 30% of all vascular plants [8]. Glandular trichomes have in common the capacity to produce, store and secrete large amounts of different classes of secondary metabolites [8,9]. Many of the specialized metabolites that can be found in glandular trichomes have become commercially important as natural pesticides, but also have found use as food additives or pharmaceuticals [10,11]. For instance, plants of the Lamiaceae, comprising species such as mint (_Mentha x piperita_), basil (_Ocimum basilicum_), lavender (_Lavandula spica_), oregano (_Origanum vulgare_) and thyme (_Thymus vulgaris_), are cultivated for their glandular trichome-produced essential oils [9]. Moreover, artemisinin, a sesquiterpene lactone that is produced in the glandular trichomes of annual wormwood (_Artemisia annua_), is used for the treatment of malaria [12]. In addition, gossypol and related compounds, which are dimeric dissquiterpenes produced by cotton (_Gossypium hirsutum_) trichomes, have strong antifungal activity [13] and are potential natural pesticides [14]. It is for these kinds of specialized metabolic properties, and for the opportunities to modify these properties via genetic engineering, e.g., [15], that trichomes have received increased attention over the past years [16]. By means of this review article, we will provide an introduction into trichome biology, thereby focusing on the biosynthesis and biochemistry of the main trichome-produced compounds, as well as their role in plant resistance. Also, we summarize some approaches that have been undertaken to engineer the metabolism of trichomes, especially those of mint, tobacco (_Nicotiana_ spp.) and tomato (_Solanum_ spp.).