The Develop module is where you edit a photo — tone and colour, cropping, retouching and local adjustments. Every edit is non-destructive: your original file is never changed, and you can undo or reset at any time.
Select a photo and press D, or click in the module bar (⌥⌘3). The photo fills the canvas in the centre, with a tool strip and adjustment panels down the right and the filmstrip along the bottom for moving between photos.
The Basic panel is where most edits happen:
Double-click any slider to reset it to its default.
At the top of the right panel is a switcher between three scopes — the histogram (tonal distribution, with highlight and shadow clipping warnings), the vectorscope (colour balance and skin tone) and the RGB parade (per-channel exposure and white balance). See Scopes for how to read each one.
Above the panels sits a row of tools that work directly on the image:
To apply one photo's look to others, use (⇧⌘C), then Paste Settings (⇧⌘V) across a selection. The History panel lists every change you've made and lets you step back to any earlier state. Nothing is ever baked into your original — Reset returns the photo to how it was imported.
Want to try a different treatment without losing the first? Create a virtual copy and edit that — it shares the original file but keeps its own edits, at no extra disk cost.