The perfection of Islam is a certainty, that a muslim must believe in, faithfully. Islamic shari’a has regulated all aspects of men’s life, for them to achieve the true happiness in life. By obeying this rules, a muslim will be able to achieve salvation and joys both in this world and in the Hereafter.
Allah The Glorified and Exalted decreed:
قُلْنَا اهْبِطُوا مِنْهَا جَمِيعًا فَإِمَّا يَأْتِيَنَّكُم مِّنِّي هُدًى فَمَن تَبِعَ هُدَايَ فَلَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ
“We said: Get you down from here, all of you, and guidance shall come to you from Me: then, whoever will follow My guidance need have no fear, nor shall they grieve.” [Al Baqarah/The Cow/2:38]
In this verse, Allah has promised salvation and joys to those who are willing to follow and obey the teaching of Allah’s messenger -peace and prayer of Allah be upon him-. Thus, it is a must to refer all problems in this life to Islamic laws, that is, of course, Allah’s law too. Also in polygamy. And a muslim is not allowed to choose any rules or laws that against the shari’a, as Allah ordained in His decree:
وَمَا كَانَ لِمُؤْمِنٍ وَلَا مُؤْمِنَةٍ إِذَا قَضَى اللَّهُ وَرَسُولُهُ أَمْرًا أَن يَكُونَ لَهُمُ الْخِيَرَةُ مِنْ أَمْرِهِمْ وَمَن يَعْصِ اللَّهَ وَرَسُولَهُ فَقَدْ ضَلَّ ضَلَالًا مُّبِينًا
“It does not behove a believer, male or female, that when Allah and His Messenger have decided an affair they should exercise their choice. And whoever disobeys Allah and His Messenger has strayed to manifest error.” [Al Ahzab/33:36]
How Islam Perceive Polygamy
In the Qur’an and Sunnah, it is mentioned that polygamy is sunnah for those who are capable of doing it. Allah revealed in His decree:
وَإِنْ خِفْتُمْ أَلَّا تُقْسِطُوا فِي الْيَتَامَىٰ فَانكِحُوا مَا طَابَ لَكُم مِّنَ النِّسَاءِ مَثْنَىٰ وَثُلَاثَ وَرُبَاعَ فَإِنْ خِفْتُمْ أَلَّا تَعْدِلُوا فَوَاحِدَةً أَوْ مَا مَلَكَتْ أَيْمَانُكُمْ ذَٰلِكَ أَدْنَىٰ أَلَّا تَعُولُوا
“If you fear that you might not treat the orphans justly, then marry the women that seem good to you: two, or three, or four. If you fear that you will not be able to treat them justly, then marry (only) one, or marry from among those whom your right hands possess. This will make it more likely that you will avoid injustice.” [An-Nisaa’/The Women 4:3]
In this verse, Allah speak to the guardians of the orphans, if they have an orphan that they are responsible of, and they are afraid that they are unable to give them enough dowry, they should turn to marry other women, since there are still more women left. Allah does not restrict his condition, because Allah has allowed him to marry until four women. If they are afraid of being wicked if they married more than one woman, they must marry only one woman, or marry their female slaves. 
By Allah’s permission, the Messenger of Allah -peace and prayer of Allah be upon him- himself had married nine women in his lifetime, as mentioned in a hadith narrated by Anas Ibn Malik -may Allah be pleased with him-:
أَنَّ النَّبِيَّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كَانَ يَطُوفُ عَلَى نِسَائِهِ فِي اللَّيْلَةِ الْوَاحِدَةِ وَلَهُ يَوْمَئِذٍ تِسْعُ نِسْوَةٍ
“Indeed, the Prophet -peace and prayer of Allah be upon him- ever went to his wives in a night, and at that time he has nine wives.” [Narrated by al Bukhari, no. 5068 and an-Nasaa-i, 6/54
Also in the saying of Ibn ‘Abbas to Sa’id Ibn Jubair:
هَلْ تَزَوَّجْتَ؟ قُلْتُ: لَا, قَالَ: فَتَزَوَّجْ! فَإِنَّ خَيْرَ هَذِهِ الْأُمَّةِ أَكْثَرُهَا نِسَاءً
“Are you married?” Sa’id replied, “Not yet.” And he said, “Get married! Because the best of this nation is one having the most wives.” [Narrated by al Bukhari no. 5069]
In the sentence of “the best of this nation” has two definitions:
First: It refers to Allah’s messenger -peace and prayer of Allah be upon him-. Thus it means that Allah’s messenger -peace and prayer of Allah be upon him- is the best person of this nation and with the most wives.
Second: It means in marriage, that is one with the most of wives.
Sheikh Mushthafa Al ‘Adawi said, “Both of the meanings share the shame basis, and indicate a similar meaning, hence becoming the foundation of the scholars who stated that polygamy is sunnah.” 
Other basis indicating that polygamy is sunnah are the hadith of the Prophet that suggest muslims to have many children.
Among the hadith are:
عَنْ مَعْقِلِ بْنِ يَسَارٍ قَالَ: جَاءَ رَجُلٌ إِلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ: إِنِّي أَصَبْتُ امْرَأَةً ذَاتَ حَسَبٍ وَجَمَالٍ وَإِنَّهَا لَا تَلِدُ أَفَأَتَزَوَّجُهَا ؟ قَالَ: لَا, ثُمَّ أَتَاهُ الثَّانِيَةَ فَنَهَاهُ ثُمَّ أَتَاهُ الثَّالِثَةَ فَقَالَ: تَزَوَّجُوا الْوَدُودَ الْوَلُودَ فَإِنِّي مُكَاثِرٌ بِكُمْ الْأُمَمَ
“From Ma’qal bin Yasâr, who said: A man came to the Prophet -peace and prayer of Allah be upon him- and said: ‘I have encountered a woman of nobility and beauty, but she cannot give birth. Should I marry her?’ He said: ‘No.’ The man then came to him a second time, but he prohibited him. He then came for the third time, and the Prophet -peace and prayer of Allah be upon him- said: ‘Marry loving women who give birth, for I will compete with you against other nations.'” [Narrated by Abu Dawud in hadith no. 2050, Sheikh Al Albani said: “good and valid”. See: Shahih Sunan Abu Dawud]
Regarding this hadith, Sheikh Musthafa Al ‘Adawi explained that: “Marriage, by Allah’s permission, may cause people increase birth. And the increase of birth can make them proud of their numbers. Thus, a fertile woman, if it is known that a man (who propose her) is infertile, should not marry him. But the prohibition (in this hadith) is makrooh, but not forbidden. Because the Prophet -peace and prayer of Allah be upon him,- kept his wives who didn’t give him children, but Khadija and Mary,” 
By that, it is known that one way to increase birth is by having more wives.
Wisdoms and Benefits of Polygamy
Every thing allowed in Islam must have it’s own wisdom and great benefits for muslims. To allow the practice polygamy is the best way to create dignified and honorable families and society.
There are several things to mention that indicate the wisdoms and benefits of polygamy, such as:
1. Polygamy is the chosen guidance, established by Allah The Mighty and The High Himself, for the sake of His slaves.
A woman might get ill, have her period, undergo her puerperal time, and the like, that interfere her to bear her tasks as wife. Whereas man is always ready to yield offsprings. Supposed a man is hampered during the time his wife unable to serve him, his functions will be lost in vain. 
2. Allah has created fewer men than women. Men also face more life-threatening events during his whole life. Supposed that each man should only marry a woman, there will be more unmarried women left, forcing them to do wicked things such as adultery and the like. And turn away from Qur’an’s guidance is the greatest source of problems in manners. 
About the number of men and women, the Messenger of Allah -peace and prayer of Allah be upon him- explained in his saying:
مِنْ أَشْرَاطِ السَّاعَةِ أَنْ يَقِلَّ الْعِلْمُ وَيَظْهَرَ الْجَهْلُ وَيَظْهَرَ الزِّنَا وَتَكْثُرَ النِّسَاءُ وَيَقِلَّ الرِّجَالُ حَتَّى يَكُونَ لِخَمْسِينَ امْرَأَةً الْقَيِّمُ الْوَاحِدُ
“Among the portents of the Hour are the knowledge will be diminished, foolishness emerge, adulteries committed undisguised, and woman’s number increases, whereas man’s decreases, until a man equals 50 women.” [Mutafaqun ‘alaihi]
3. Generally, all woman is always ready to marry, but in contrast, plenty of men do not have the ability to bear the consequences of marriage due to their poor condition. Thus, the number of men ready to marry is less than woman.
4. Polygamy will be a mean to elevate the status of a woman whose husband is dead or divorced her, whereas she has none of her family to be responsible of her. By polygamy, there will be a man who is responsible for her needs.
Beside that, polygamy also offers a lot of benefits toward individual, the society, and muslim communities as a whole. Among those benefits are:
One of the most effective way for people to lower their gaze, maintain their dignity, and multiply their offsprings.
5. One of the way to protect men and women from various factors of wickedness and deviation. Sheikh Ibn Baz said in his fatwa, that polygamy contains great benefits to men and women as a whole. Because, by polygamy, All people will obtain benefits; lowering of gazes, maintenance of dignity, plenty of descendants, men will be able to do much for the sake of his wives, protect them from factors of wickedness and deviation of manners. 
6. Increasing the number of muslims, thus Islam will have enough human resources to face their enemies in holy battle.
Sheikh Muhammad Al Amin Asy Syinqithi said, “Qur’an legalizes polygamy for the sake of women, to be able to get husband, and for the sake of men, to protect their uses from being wasted, such as when a woman gets older, and for the sake of muslims to increase their numbers, to be able to face their enemies to establish and keep Allah’s enjoin high.” 
That is the beauty of Islam, regarding it’s permission for polygamy. However, to practice it, one must fulfill certain criteria and requirements stated in shari’a. And Allah knows best.
[Copied from As Sunnah magazine, 12th edition/Year X/1428 H/2007. Published by Foundation of Lajnah Istiqomah, Surakarta, Jl. Solo – Purwodadi Km.8 Selokaton Gondangrejo Solo, Indonesia 57183 Telp. (62) 0271-761016]
Author: Ustadz Abu Asma Kholid Syamhudi
Article of www.whatisquran.com
. Shahih Fiqhus-Sunnah, author: Abu Malik Kamal bin as-Sayyid Salim (3/215-216).
. Jami’ Ahkamun-Nisaa’ (3/441).
. Jami’ Ahkamun-Nisaa’. (3/442).
. The saying of Sheikh Muhammad al Amin asy-Syingqiti in his work “Adhwa’ul-Bayan” (3/377), cited from Jami’ Ahkamun-Nisaa’ (3/443-445).
. Jami’ Ahkamun-Nisaa’ (3/442).
. Shahih Fiqhus-Sunnah (3/217).
. Al Fatawa asy-Syar’iyyah fil-Masa’il al-Ashriyyah min Fatawa Ulama al Baladil-Haram.
. Al Fatawa asy-Syar’iyyah fil-Masa’il al-Ashriyyah min Fatawa Ulama al Baladil-Haram (3/442).
. Taken from: Jami’ Ahkamun-Nisaa’ (3/446).