Because of their internal structure, molecules provide novel functionality not realizable in conventional semiconductor-based electronics. One exciting new possibility is that of spintronics: electronic devices using the electron spin to carry and manipulate information. So far, spintronics has been explored in metals and semiconductors. Magnetic molecules in principle enable radically new approaches in using the spin degree of freedom, but their incorporation in solid-state devices is a daunting task. In particular, the main challenge is to control their spin for storing and reading information. We propose to use electric fields and light for this purpose.Based on our recent breakthroughs in making nanoscale junctions of noble metals and graphene, we will fabricate and study planar spin transistors built up from individual magnetic molecules or nanoparticles. A key device feature is that electrodes are separated by a distance on the scale of the molecular object itself. This geometry allows for in-situ application of strong local electric fields as well as optical fields to modify magnetic states and hence influence the conductance.The objective of this proposal is to study how the electric conductance through single molecules and nanoparticles can be used to probe their magnetic properties and how external stimuli can control them. We will perform proof-of-principle experiments divided into four challenging tasks: 1) Study of quantum aspects of transport through single magnetic molecules and nanoparticles; 2) Room-temperature studies of molecular magnetism on the molecular scale; 3) Measurement of spin-polarized currents through molecular-scale magnetic junctions; and 4) Control of molecular magnetism by local electric and optical fields.By obtaining a detailed understanding of the interplay between molecular magnetism and transport we strive to establish new strategies towards in-situ spin-state control and the development of novel spintronic nanodevices.