I looked them up in the dictionary and they both seem to refer to something that needs to be done. For example, I feel that all the sentences below are grammatical. What are the differences between the two words? - Teachers have an obligation to treat all their students fairly. - Teachers have a responsibility to treat all their students fairly. - Janet will have responsibilty for the new advertising campaign. - Janet will have an obligation to run the new advertising campaign. Thanks for your help!