A Special Summon can only take place when a card effect allows you to. Normally the card effect will specify a place that the monster being Special Summoned will come from. For example, "Special Summon from your Graveyard".

If it is not specified, usually you can only Special Summon it from your hand. These are effects like "This card can only be Special Summoned by". If it is a "can only be Special Summoned" effect, it cannot be Special Summoned from the Graveyard (Monster Reborn) or remove from play area (Return from a Different Dimension) unless there was an attempt at summoning it in the past when the actual requirement on the card was met, and this original Special Summon had not been negated. If it is a "cannot be Special Summoned except by" effect, then you can't Special Summon it unless you do exactly what it says. Again, the card must be in your hand unless it states otherwise.

Conversely, the card Polymerization does not explicitly state a "from" location, and its text is very vague. In this case, you Special Summon a card from your Fusion Deck (called a Fusion Summon).