Click the first photo, then hold Shift and click the last photo to select a range
Hold Command and click individual photos to select non-adjacent photos
Click and drag to draw a selection box around multiple photos
In Finder, press Command + A to select all photos in the current folder
In Photos app, click one photo, then hold Shift or Command while clicking others
To deselect a photo, hold Command and click it again
Use Shift + Arrow keys to extend a selection in some views
Use Command + click to add or remove photos from your selection