Biodiversity within the world’s oceans is a virtually untapped resource for the isolation and development of novel compounds, treatments and solutions to combat human diseases. Marine environment has enormous source of compounds with uncommon and unique chemical features providing therapeutics of greater efficacy and specificity. In spite of the advancement in biomedical research and technology, cancer continue to be a great threat to public health. This points to the urgent need for the development of new anticancer drugs. The marine organisms have revealed to be one of the major sources of new compounds, including those exhibiting antitumor potential. These compounds also demonstrated ability to mediate specific inhibitory activities such as angiogenesis, apoptosis pathways, migration and invasion, in both in vitro and in vivo models. The present chapter will focus on the bioactive molecules from marine organisms with a focus on how these compounds act upon the hallmarks of cancer.