It is notoriously difficult to observe, let alone control, the position and orientation of molecules because of their small size and the constant thermal fluctuations that they experience in solution. Molecular self-assembly with DNA provides a route for placing molecules and constraining their fluctuations in user-defined ways and with up to Angstroem-scale precision. These positioning options open attractive and unprecedented avenues for scientific and technological exploration. In my talk I will introduce some of the key concepts and methods, and highlight a number of recent developments. In particular, I will talk about creating complex molecular mechanisms and the ability to perform precision measurements of biomolecular structures and interactions using custom DNA origami devices.