Here is a brief, non-exhaustive review on some of the things I found to be helpful when trying to elucidated extracellular and intracellular signaling. Click on each page to see them better. Although I am particularly interested in immunology, these concepts apply to all cell/molecular biology and can be used when studying signaling in the nervous system, endocrine system, digestive system, etc. The images are not my work and are borrowed from other websites. A list of useful websites are also presented in this article. Please write any comments/suggestions/corrections that you may have.