Plants are eminent source of new therapeutic agents that helps to alleviate human ailments and promote health. The noteworthy preventive and protective properties of these substances are related to their strong antioxidative, antimutagenic and anticarcinogenic potential. Among these, alkaloids are important secondary metabolites that are known to possess curative properties and are of prime importance for humankind. On the basis of their biosynthetic precursor and heterocyclic ring system, the compounds have been classified into different categories including indole, tropane, piperidine, purine, imidazole, pyrrolizidine, pyrrolidine, quinolizidine and isoquinoline alkaloids. These are important therapeutic molecules due to their efficacy to prevent the onset of different degenerative diseases by scavenging the free radicals or binding with catalysts of the oxidative reactions, such as some metal ions. These molecules also inhibit the growth and development of microorganisms including bacteria, fungi, protozoans etc. Due to their immense properties, these compounds are in great demand for pharmaceutical formulations and might emerge as valuable metabolite used to cure many lethal diseases like cancer. In this review, we aim to discuss about alkaloids, their biological activities and related mechanism of action for protective behaviour.