Looks like tomato blight. Remove and destroy (don't compost!) the worst of the damage. Next year plant resistant varieties and don't plant in this spot for a few years if you can.
There's not a whole lot you can do but many tomato plants will outgrow it and continue to produce. You could try spraying with a fungicide like copper, but in this heat it might do more damage than good.
It's a fungus -- there are actually two called late blight and early blight, but they can show up anytime. The spores are mostly in the soil. In humid areas it's pretty hard to do anything about them. You can help prevent it by never watering overhead and not over-watering, by providing good air flow around your plants by spacing more widely and pruning, trimming up the lower branches and leaves so nothing touches the ground, rotating your crops and avoiding excess nitrogen. Also some people suggest mulching to prevent dirt from splashing up onto your plants in a heavy rain, but I think trimming the lower bits does most of that.
If you a gardener that uses sprays and such, you can start anti-fungal sprays early and repeat per the label instructions. Remove any signs of disease immediately.
I see a little bit every year but nothing like your poor tomatoes. Really nothing this year since it's been so dry down here.
I would check some of the tomato blight pictures online with close-ups of the leaves and damage to verify that it matches your symptoms.