Thank you, Bruce. It sounds like a very simple solution, and though it took me a while, I figured out where the Get Count tool is and I think I was able to make it a precondition -- never used either technique before. When I connected the Row Count to the next step in the process, I chose Precondition from the context menu. I assume that's how I'm supposed to do it. Unfortunately, even without a selection in my Layer, the Dissolve tool still ran in this example. I'm sorry to ask, but can you take another look and see if I've missed something? I'm still quite at amateur level with Model Builder.
Thanks!
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Edit: Thinking about this further, and looking at the Python generated from the model, I can see that there's no check for "number of rows SELECTED" in this. I'm not sure how to do that part with the Get Count tool. Can you give me specifics as to how you recommend that part is done? Much appreciated!