excel summary sheet

i have a workbook with say seven sheets. Each sheet contains data for seven different companies. All sheets are in the exact same format. Assuming on a monthly basis that some sheets have say 10 lines of data and others might have 15 etc how can i do a summary sheet that puts all these lines of data onto the one summary sheet, remembering that the amount of lines contained on each individual sheet will change each month? I want to be able to do this without physically copying and pasting each month.