Bridge has evolved over the years and has become a bidder's game.  When the opponents open the bidding,
a bid by your side is an termed an overcall.  Overcalls come in many flavors. 

An Simple Overcall is made at the cheapest level available. An overcall on the 1 level promises a minimum of 8 points and a good 5 card suit.   If you have a weakish suit you need a good hand to justify your overcall.
A simple overcall usually limits your hand to 16 Points.  With more strength you would double first and then bid your suit at your first opportunity.  

If you overcall on the 2 Level your promise more values.   If you are overcalling with a 5-card suit you need opening values -- if you suit is longer -- you can overcall with fewer points.

Jump Overcall -- A Jump Overcall is weak -- promising values similar to a Weak Opening at the level you bid.

1NT Overcall should show 15+ - 18 and promise stoppers in the opponent's suit.
Note:  I prefer "systems on" for advancer when partner overcalls 1NT.  Discuss this with partner.

Two Suited Overcall  -- Conventional Bids Like Michaels and the Unusual Notrump
Reasons to Overcall  
  • Compete for the Auction
  • Obstruct the Opponents' Bidding
  • Provide a Lead
  • Suggest a Sacrifice
Since partner doesn't promise an opening hand when he overcalls -- advancer must  use judgement.   The most important criterion is how good a fit do you have with your partner.  Another important factor is the nature of the auction.  Did your RHO opponent bid?  And if so what?  Look at the vulnerability and do the math.
Did the opponent's find a fit, and if not -- what is your holding in Opener's suit.  There are many factors to consider. 

Raising Your Partner Overcall when you have a Fit
  • With 3 of the suit and 6-9 points give an immediate raise
  • With 4 card support and a weak hand you can jump in the suit partner has bid. *
  • With 4 card support and a limit raise -- cuebid the opponents' suit
  • With 5 card support  -- a jump to game is preemptive.
If you have a relatively weak hand and no fit it is usually best to pass.  If you have decent hand you can bid a new suit.  Partner will not find this particularly encouraging.  On the one level it is forcing for one round.  On the 2 level it is a matter of partnership agreement -- if partner is a passed hand I suggest you play this as non-forcing. Knowing this, partner should think carefully before he introduces a new suit on the two level. 

When your partner bids and your RHO passes and you hold a balanced hand with 2 of your partner's suit -- you might consider bidding notrump in hopes of improving the contract.  If your RHO does enter into the auction proceed with caution. 

Guidelines for Bidding Notrump in Response to An Overcall by your Partner
  • 1NT 8-11 points
  • 2NT 12-14 points
  • 3NT 15 or more points