Companion Planting....Science or Folklore?

Both really.....depending on how you define it. Often described as 2 or more types of plants grown in close proximity that have a symbiotic relationship.
The science parts that stand to reason:

• some plants can create helpful shade for others
• some plants can provide a windbreak to others
• some plants can use others as a trellis
• legume plants actually 'fix' nitrogen into the soil
• some vine crop plant can provide "ground cover" for others, but also competing for water and nutrients along the way 

The folklore tales etc:

• tomatoes make my carrots taste better (really?)
• Marigolds deter aphids (really? how many did it chase away?) 
• potatoes and tomatoes won't grow side by side (funny - they are both from the nightshade family)

Many things are hard to measure. Take the insects for instance; Aphids do not winter over here, they blow in from year to year, so they aren't even present every year in our climate.
I am more of believer in how plants can be grown to benefit each other like growing annual rye or legumes as plow down crops to benefit next year crops. Or to plant alfalpha near a strawberry field, because the tarnished plant bug will chose alfalpha over strawberries. Or to plant corn where you had legumes last year due to increased soil nitrogen.