Unripe tomatoes represent an agri-food waste resulting from industrial by-processing products of tomatoes, yielding products with a high content of bioactive compounds with potential nutraceutical properties. The food-matrix biological properties are attributed to the high steroidal glycoalkaloid (SGA) content. Among them, α-tomatine is the main SGA reported in unripe green tomatoes. This review provides an overview of the main chemical and pharmacological features of α-tomatine and green tomato extracts. The extraction processes and methods employed in SGA identification and the quantification are discussed. Special attention was given to the methods used in α-tomatine qualitative and quantitative analyses, including the extraction procedures and the clean-up methods applied in the analysis of <i>Solanum lycopersicum</i> L. extracts. Finally, the health-beneficial properties and the pharmacokinetics and toxicological aspects of SGAs and α-tomatine-containing extracts are considered in depth. In particular, the relevant results of the main in vivo and in vitro studies reporting the therapeutic properties and the mechanisms of action were described in detail. In addition, a comprehensive overview of the biological effects of unripe green tomato extracts containing α-tomatine, pure α-tomatine, and its aglyconic part, tomatidine, is reported, showing their potential use for human health and dietary supplements. 2. Unripe Green Tomatoes: A Source of Glycoalkaloids SGAs are a class of secondary metabolites produced by some plants in the Solanaceae family, such as tomatoes, potatoes, and eggplants. They are distributed in different botanical parts, which include the leaves, flowers, stems, roots, and unripe fruits [ 20 ]. Tomatine was firstly isolated by Fontaine et al., in tomato leaves and represents the main SGA isolated from the whole tomato plant [ 21 ]. Tomatine was originally isolated as a mixture of two SGAs, α-tomatine and dehydrotomatine, which differ based on a double bond in the aglycon moiety across positions 5 and 6. However, the chemical composition of tomatine commercial standards were characterized as a mixture of 90% α-tomatine and 10% dehydrotomatine in 1994 [ 22 ]. Therefore, all the studies conducted up to that year on tomatine actually referred to a mixture of the two compounds, presumably with the ratio 9/1. During the transition from green to red fruit, it is metabolized to esculeoside A, which represents the main mature tomato SGA [ 25 ]. As described in the literature, the α-tomatine concentration in tomato plants is highly variable, and different quantification values are reported. Some authors have identified the fruit as the botanical part with the highest SGA concentration [ 27 ]. Instead, other authors have reported a higher α-tomatine concentration in the stems and leaves [ 26 ]. It is thus not surprising that the healthy properties of green tomatoes’ alkaloids are a consequence of the antibiotic power of these secondary metabolites. Leaves and immature green fruit extracts of Californian Solanum lycopersicum L. display antimicrobial activity against several bacteria ( Salmonella enterica , Staphylococcus aureus, and Escherichia coli K12) [ 81 ]. Interestingly, the extract does not affect the growth of the beneficial bacteria Lactobacillus acidophilus , Lactobacillus rhamnosus , and Lactobacillus reuteri , which are part of the human gut microbiota [ 81 ]. Extracts obtained from the locular gel and serum of Solanum lycopersicum L. var. “Camone” (respectively, containing 61.7 ± 0.9 mg of α-tomatine/kg of FW of locular gel and 12.5 ± 0.5 mg of α-tomatine/kg of FW of serum) significantly reduce inflammation in humans, decreasing the blood inflammatory cytokine count, systolic pressure, heart rate, and aorta thickness [ 46 ]. The supplementation of 1–2% dietary tomato powder containing α-tomatine ameliorates hemato-immunological and antioxidant clinical parameters in rabbits [ 86 ]. Although pure α-tomatine has proven its anti-tumoral effect in many studies, dehydrotomatine is reported to have no effect in the reduction of cell proliferation [ 52 ]. 5.5. Pharmacokinetics and Toxicological Aspects of Glycoalkaloids and Green Tomato Extracts The correlation between in vitro pharmacological activities and possible beneficial effects on human health is strictly correlated to the study of α-tomatine pharmacokinetics (e.g., absorption, distribution, biotransformation, and excretion). As is well known from the literature, phase I and II metabolism reactions generally increase a drug polarity’s to accelerate its elimination rate in urine. Therefore, it is reasonable to hypothesize that sulfated and glucuronidated metabolites are involved in α-tomatine excretion. Some studies on other SGAs have detected some sulfated metabolites in human urine. One example is represented by esculeogenin B sulfonate, which derived from the metabolism of esculeogenin B that is present in ripe tomato products such as juice [ 121 ]. Although these studies partially described the in vivo α-tomatine metabolism, unfortunately, to our knowledge, there are no complete studies focused on the absorption, distribution, and excretion of this molecule. Moreover, these studies were performed on mature red tomatoes products with a low α-tomatine content, while no published data have Meanwhile, the accurate correlation between the beneficial effects and concentration of active ingredients through a harmonized and standardized quantification approach is a requirement. Therefore, additional properties (e.g., nutraceutical and pharmacokinetic in vitro effects) of tomato SGAs and Solanum lycopersicum L. extracts will require further development to investigate their effects on humans in proper in vivo studies. Figure 1 Chemical structure of α-tomatine. Structure was drawn using the chemistry software ChemDraw Professional 15.0. Figure 2 Scheme of biomedical effects of green tomato SGAs and extracts.