Abstract: The field of cosmology has had great successes in the past decade yet there are many outstanding questions. Gravitational lensing will be an instrumental tool in our quest to answer these questions. Lensing is being used to probe the distribution of matter over a huge range of size scales - from individual stars and planets to fluctuations in the density of the universe that are hundreds of millions of light years across. These studies have implications for the properties of the dark matter particle, the theory of gravity, the reionization of the universe and the formation of galaxies. Our current understanding of how structure formed in the universe will be outlined and it will be shown how gravitational lensing will shed further light on it.