This is the fungal disease Septoria. It very common on tomatoes, and the management is similar to early blight, another common fungal leaf spot. Here is a list of management practices on our website: http://extension.umd.edu/hgic/septoria-leaf-spot-vegetables
Search for 'early blight', too, on our website search.
In wet years, it is harder to keep up with these problems.
If you do not have too much infection yet, pull off the heavily infected leaves before you apply a fungicide. If they are heavily infected, you may want to consider replanting. It's still early in the season.
When mulching to keep spores from splashing onto plants, layer newspapers 3-4 deep and overlapping, under the straw or other mulch, to form a solid barrier to the spores.