Aphids are small, soft-bodied sap-sucking insects, usually green, black or grey, that cluster in tight groups on new growth, flower buds and the undersides of young leaves. They're easy to tell apart from most other small garden pests once you know what to look for: aphids barely move when disturbed and cluster densely rather than scattering, and are often accompanied by ants, which farm them for the sweet honeydew they excrete. Whitefly, by contrast, scatter into the air in a small cloud when a leaf is brushed, which aphids don't do.
New growth is where aphids concentrate because it's the softest, most nutrient-rich tissue on the plant, and their piercing mouthparts work best on tender cells rather than tougher, more mature leaves. A few aphids on a new shoot tip is common and rarely worth acting on. It becomes a problem once colonies are dense enough to visibly curl or distort leaves, stunt new growth, or coat leaves in the sticky honeydew that then grows black sooty mould on top of it.
For light to moderate infestations, a strong jet of water from the hose, aimed at the undersides of affected leaves, physically knocks most aphids off and is often enough on its own, repeated every couple of days until numbers drop. Insecticidal soap sprays are the next step up: they work by disrupting the aphids' waxy outer coating on direct contact, so thorough coverage, including leaf undersides, matters more than the product itself, and they break down quickly so they don't build up in the garden.
Ladybirds and hoverflies are the two main natural predators worth encouraging rather than fighting against. A single ladybird can eat around 50 aphids a day as an adult, and its larvae are even more voracious over their development. Hoverfly larvae are also serious aphid predators. Both are drawn in by a steady supply of flowering plants through the season, rather than one type flowering all at once, since the adults feed on nectar and pollen even though their larvae are the ones eating aphids.
A single ladybird can eat around 50 aphids a day as an adult, and its larvae are even more voracious over their development.
Nasturtium is worth a specific mention here: it's a genuine trap crop for aphids, as well as caterpillars and whitefly, meaning it draws these pests onto itself and away from nearby vegetables, and it's a good companion for tomatoes, cabbage, lettuce, peas and spinach among others. Planting a few nasturtiums near aphid-prone crops gives you both an early warning sign, check the nasturtiums first, and a decoy.
Tolerate rather than treat if numbers are low and predators are already present. Spraying at the first sign of any aphid removes the food source ladybirds and hoverflies need to establish, which can make the underlying problem worse over a season even if it looks tidier in the short term.