# In an isothermal process work is done on/by the system (expansion or compression of the gas) yet still the internal energy remains constant, why?

An isothermal process indicates that $\Delta Q = 0$ as there's no change in heat energy. But in most text books it's taken as $\Delta Q = 0$ and internal energy change is taken as a constant. At the same time a 0 change in heat energy implies a change in work done on/by the system and, by definition work done by/on the system contributes to internal energy $U$ right?

An isothermal process is not necessarily one in which Q = 0. In an isothermal process, the only thing we can say is $\Delta T=0$.