Cannot find module 'express'

Hi while running 
`node app.js`
I get following error.

`module.js:549
    throw err;
    ^

Error: Cannot find module 'express'
    at Function.Module._resolveFilename (module.js:547:15)
    at Function.Module._load (module.js:474:25)
    at Module.require (module.js:596:17)
    at require (internal/module.js:11:18)
`